buying winter clothes in summer?

Will it be difficult to find winter clothes (scarves, jackets, etc) in August?

I think that most clothing shops at that time will have mostly summer clothes on sale. However, expensive luxury labels usually start selling fall/winter clothes in July (as they do in major cities all over the world). Of course, stores like this are not in most people's budgets, and it would most likely be the early fall clothes, with the winter clothes being introduced a little later.

I'm not sure on this, but it is possible the outlet mall in Gotemba (if you will be in or near Tokyo) will have past seasons of winter clothes. I do not want to disappoint you if they do not however, so I am using caution on this.

So many Japanese like things like Gap and Banana Republic and what not, which I'm sure you can get in your own country, right?

Yes, it will be difficult- other than at an outdoor shop selling mountaineering gear, shops will only just be starting to change over to autumn clothes, which won't include thick coats or warm scarves.

http://www.premiumoutlets.co.jp/en/gotemba/

it is about an hour and a half one way from Shinjuku Station in Tokyo. more specific directions are on the site

again, they may not have any winter clothes at the time you go, but I always find this place fun