Finding an accommodation in short notice

So I want to plan a trip across Japan and want to be flexible with my destinations, so if I want to stay somewhere for a day or two longer I would just change my travel route slightly. Therefore I do not want to book the hotel, hostel, AirBnB, etc. in advanced but like 2-3 days beforehand.
So my question is, if I even can find rooms in that short amount of time without paying like 100€ per night or is there a lack of affordable rooms and I have to book beforehand. My travel time is probably around July/ August 2022 so not during Sakura.
Has someone experienced something like this or knows how it is besides the pandemic, would be happy if someone could help me with my worries :D

It depends on the place and time. A lot of the time it is no problem, but there are certain locations during certain times that can get nearly or fully booked out, especially during busy travel seasons or special events. One such busy travel period is the Obon period in mid August. And leisure travel is generally intensive during the summer holidays from late July through August. Hotels in business districts, on the other hand, tend to be less busy during these times. Big cities like Tokyo and Kyoto with a huge number of hotels are unlikely to be a problem. But holiday resorts in the mountains etc. may be a little bit more challenging.

Just use a booking website.

Or use a Japanese hotel company english website like Toyoko Inn.

If you want to maximize your savings. Dont make changes on Friday and Saturday nights.

I traveled 2 months through Japan by bicycle and public transport and normally only booked the next day accommodation a day or two in advance.

http://bicycletraveljapan.blogspot.com/search/label/Accommodation+Prac...

I generally used Jalan webpage for booking. You need to be a bit flexible but I never had issues. But as PPs said Obon is a busy period.

Many hotels will also allow for really late cancellation, eg the day before. But if you go that route, make sure you remember for each hotel THEIR specific cancellation policy so you don�ft cancel too late.

If you have any flexibility in your dates and if you aren�ft super humid-heat resistant, try NOT to travel in July and August.

100euro is around 13,000 yen, so you are already looking at cheaper/budget accommodation. Getting that all the time at short notice you might be spending extra to travel to some place and the quality may not be high. (as always, trade-offs)
I recall seeing a programme about some guys travelling at short notice like that and they ended up paying 30,000 yen for a crappy room in Tokyo. Similarly, there was a thread here a few years back where someone got a cheap flight to Japan during their long SE-Asia holiday and they were then pissed that accommodation in Japan was much higher than other countries. It is possible to do things at short notice and I have done it on occasion, but usually to places I have been before so I know what I am getting.
Do some research on the hotel booking sites - see what you can get for next week in various places around Japan - that will give you an idea of availability and price. Toyoko Inn might be a good option for you (cheap, clean, small), but I have found them to sell-out months in advance too.

I have changed travel plans on the fly many times without issues. It helps to know the cancellation policy of each room you reserve. Generally speaking, it's easier dealing directly with the hotel, rather than through a third party reservation service.

Given the excellent cancellation policies for most accommodation in Japan, I would make bookings in advance that you can cancel for free the day before. That way, you have somewhere to stay if you decide to follow your original plan, but can cancel on short notice if your plans change.

I would be hesitant to travel without any bookings in place, as you are more likely to face higher prices, wasting your travel time looking for accommodation, having to stay in less appealing locations, etc. Have some plans in place at the very least.

but can cancel on short notice if your plans change

Just don't make reservations that you do not intend to honor. Otherwise the excellent cancellation policies will soon be history.

There are a couple of ways to travel around Japan.

First - 100 Euro a night on average should easily be able to find you simple accommodation for most nights in Japan. That's roughly 13,000 yen a night for a single person.

Your first problem with looking for accommodation at the moment is that many hotels only allow you to book up to 6 months in advance. Friday/Saturday nights also tend to be much more expensive - specifically if it's a more touristy hotel near the beach.

I've visited all prefectures and in general tend to spend around 5-8,000 yen a night for a simple business hotel that includes a clean bathroom. Note that secondary cities are much cheaper. Booking accommodation for slightly longer times in advance (say 3+ months and 4-5+ nights) can save you a lot. For example - I stayed at a 3.5 star hotel in Kyoto for 5,300 yen a night in January 2020 for 5'ish nights. In autumn or cherry blossoms pre-covid - this hotel might be 20,000 yen a night in peak season.

Japan has a lot to do in some areas where it's possible to complete a lot of day trips. For example - Kyoto/Osaka could be a base for a week - the same is true to Tokyo. Having a base and making day trips is a good way of visiting Japan for first timers.

Uji makes a good point on not abusing good cancellation policies. The only one I used to do this with is Toyoko Inn - but I've not stayed in one for years. The websites I find most useful include:

https://travel.rakuten.com/
https://www.jalan.net/en/japan_hotels_ryokan/
https://www.japanican.com/?cid=1839029 (this one is often more expensive when it's a normal booking - but had exclusive specials which were sometimes good.

booking, expedia and agoda also had lots.

Ok thanks guys for the detailed explanations and recommendations it really helps me a lot :)