Ask around and try to find a good seamstress or tailor. It's unlikely you'll find this person at your local dry cleaners. A good seamstress may be able to make an OK jacket fit near perfectly so make sure when buying or thrifting that you figure the cost of alterations into the cost of the clothes. An excellent seamstress will not only do good work, but give you good advice on what adjustments can be made and what are not worth the money due to the high likelihood of failure.
Thanks and will do. I'm in boston, so there shouldn't be a shortage. The shortage is with my $$$ however and the 9-10 odd blazers I have, I've probably spent 100-150 on total. As I said earlier, most are of acceptable fit with a few being more off. That said, there's no point sinking money into something that's going to stay acceptable.
Thanks for all your help everyone. I'm going to stop derailing this thread now.