I was extremely lucky when I found my montrose doublet on eBay and it fit. As everyone points out, they are pretty much bespoke (tailor made). And, they are indeed close fitting. I was afraid the thing would be too short, but I had good contact with the seller. So, I knew it would be sized pretty close to what I would get.

The Sherrifmuir is the best bet if you are looking for a very formal doublet and don't want to spend money. As previous poster pointed out, J. Higgins has them for a good price.

Of course, we are talking extremely formal clothing. I would say more white tie (i.e., Fred Astair tailcoat) than black tie. Most people can get away with a prince charlie coat or a black argyll jacket.

In fact, the black argyll jacket is the most versatile of all in my opinion, if you have to get just one jacket.