People spend so much money on custom suits these days and personally I get it but do not do it myself because of the cost.
What I have done a few times is buy a real nice get up from the thrift store. Know your shoulder size or just try it on to see it if kinda fits. Shoulder size is the most important part of the process since this is the hardest part to tune up.
What are you looking for in a nice blazer? Anything really, depends what you are into. Are you one of the asshole that flashes the brand on the inside pocket or do you want a nice natural breathable materiel? Your choice but I would look for a nice virgin wool suits or just nice fashionable blazers with skinny lapels.
OK, so you found one. Your next stop is the tailor. Head to your corner store tailor and see what he can do. If he fudges it up, it was only 10 bucks tops just make sure to try it on before you leave his shop.
For a $10 blazer and a $40 fitting job, you now have a suit that fits better then those crap mail order custom suit shops. Boom.