There are many different types of any clothes from Marvel, like anything you can get now, and hopefully I’ll get more marvel clothes for Christmas or even my birthday and like I could go on and on about the different types of Captain America clothes with its own design but that would take way too long so I’m going to cut to the chase and pick just captain America t-shirts and some clothing extras such as belts etc. for this occasions.
A great site for buying any type of super hero stuff is ironically superherostuff.com, once you get to the Captain America t-shirt page (which I think the navigation is very simple to use), you will find a large selection of pretty good Captain America t-shirts and actually a great range of Captain America clothing too.
At the start the t-shirt are a bit repeated and basically the same but near the bottom I think it gets a lot better (like each of the Spiderman movies). This clothing comes in a lot of size and is reasonably cheap but this site is American (I know the irony!) and therefore operates on the dollar basis not great for a non-American buyer and the postage costs are also quite expensive unfortunately.
I also love the idea of a Captain America belt buckle but after search for like ages I have concluded that there is not many made but some can be found on superherostuff.com, amazon.com and ebay.com but ebay.com would actually have the biggest and best selection.