Obviously, there isn't a Goodwill on Savile Row. But this interactive graphic from NPR on the difference between a $99 suit and a $5,000 suit might give you the knowledge and the confidence you need to find that gem that just needs a little polishing. Also, it helps to hunt for bargains in locations where rich people are likely to donate high-quality, lightly-worn clothes (like Washington, D.C.).
If you're in the Greater D.C. area, the Goodwill at 10 S Glebe Road, Arlington VA 22204 gets all sorts of good stuff from folks who move into the area only to discover that their townhouses can't hold all their stuff.
Update: this link at lifehacker.com entitled "Use These Handy Tests to Make Sure Your Suit Fits Right" might come in handy as well.
Happy bargain hunting!