i'm in the same boat... I'm carrying a few extra pounds, so I can't shop in the H&Ms of the world...
note this advise is based on what is in the greater NYC area, most of these places/brands might be available online, so look and see what works for you.
if money is an issue, you can do it 'on the cheap' at stores like Daffy's, Marshall's TJ Maxx, etc will have a few nice pieces possibly in the right size.
if money isn't an issue, but you don't want to spend a TON, a lot of the 'urban' clothing stores such as Against All Odds, Stylz, etc will have some larger sizes in brands like Parish, Marc Ecko, Avirex, Johnny Blaze...
if money is really no issue, Rochster Big and Tall has some great clothes from real designers like Calvin Klein, Burberry, Joseph Aboud, Kenneth Cole, etc. (Casual Male XL and He-Man Big and Tall carry bigger clothes, but it looks Kmart/Wal-Mart fashion) . The clothes here might not be overly peacock worthy, but as a bigger guy, having a pair of jeans that looks good and fits you well is enough... i've found that wearing clothes that LOOK good on you adds a level of confidence that acts in much the same way...
http://www.rochesterclothing.com/store/en_US/index.jsp
and if money is TRULY no object, Rochester stores (not online) will custom make suits and shirts for you, and you can make them in any style/color, etc to create your own look.
i alternate between a nice looking suit with a flashy t-shirt, nice jeans, nice shirt, sport jacket and the occasional over-the-top outfit...