Are you trying to find an authentic U.S. Navy issue pea coat or just a copy from a clothing store? I've just recently started collecting U.S. Navy issue pea coats so I'm not quite an expert on them yet, but I'm getting there. I only have two in my collection so far but am working on getting more.
Anyway, if you want to go genuine you'll want a recent one, as they are the only ones that are black. The older vintage ones are navy blue. You can find them on Ebay or at your local Army/Navy store. If you don't care whether it is genuine or not then a lot of clothing companies make replica pea coats. It's difficult to find 100% wool coats, though, outside of the Navy issue ones. Most replicas are a wool/nylon blend. One merchant even tried to tell me that you can't get 100% wool anymore (I called him on that one real quick...). What is your price range? You probably won't find a decent one for under $100. Write back with more info on what you're looking for and I'll try to point you in the right direction.
Oh, and pea coat sizes (at least the genuine ones) are the same as suit coat sizes. So an x-small would be a 34.
EDIT: Actually based on your brother's height and weight I would recommend a 36 or a 38. I only know that because he's the same height and approximately the same weight as I am.