Alternatives of JR Pass

Hi! Instead of buying the JR Pass, I want to buy a round-trip train from Tokyo to Hiroshima and buying separate supplements tickets to stop in some cities along the way.

Where can I buy the fare ticket plus the supplements? Just at the station or can be bought online?

You can buy the tickets at ticket offices, selected ticket machines or online. It is explained below:
https://www.japan-guide.com/e/e2018.html#ticket

Correction: I suspect that the Tokaido Sanyo Kyushu Shinkansen Online Reservation Service, the English online reservation tool, cannot be used to purchase supplement fees separately. But you can do so at offices and selected machines (the ones that allow for seat reservations).

Note that depending on the JR company, the supplement ticket is usually called "limited express ticket" or "express ticket".

Bad news: I went to a ticket machine just now to check, and the English interface of at least JR East ticket machines does not seem to be set up to allow for base far ticket purchases. So, I think you will have to go to a ticket office to purchase the tickets in the way you intend.

Thank you Uji for your answer!

The problem is that I want to know if this option is the best, maybe I need a calculator with base fare tickets + how the supplemets cost.

It is definitely cheaper the way you intend. You can look up base fares and supplement fares here:
The website calls the base fare "ticket" and the supplement fee "Reserved seat" (or "Unreserverd seat" I suppose).

it is cheaper of 2376 yen plus alpha. plus alpha depends on how many time you stop. but, I believe that it is less than 100 yen for one stop. the reason is that when you buy one basic fare ticket, you pay one initial cost. (if you buy multiple tickets, you need to pay initial costs multiple times.)
2376 yen = 11880 yen(one way basic fare) X 2 (round trip)X 0.1 (discount rate for a round trip)

the difference is not much, but beneficial.

You don't specify what other cities you want to go to, or for how long. You might consider getting the Kansai-Hiroshima Pass if you want to do several stops in the Chugoku Region, and get regular fare tickets for the Tokaido Line.

Hi!!!
I see should be cheaper.

I'm willing to stop in the next cities along the way between Tokyo and Hiroshima.

- Odawara
- Osaka
- Kyoto

Not sure if I'm going to stop from Tokyo to Hiroshima or from Hiroshima to Tokio. I still figuring out.

Thanks all for your help!!!