I usually post this in all the "where do I buy a suit" threads. Warning; I've become a clothes fanatic over the past year, spent a lot of time reading up and doing research, and am still by no means an expert. If you consider this a waste of time, going to your local menswear store or a department store will be easier and less taxing, although less likely to yield great finds.
First, get your measurements taken (learn your size, but your actual measurements are what matters, because sizes vary). You can do this at any local menswear store. If you have a high end men's clothier, try on some top of the line suits. Right away you should feel a difference in how soft the fabric is, as well as the general fit. Also, read
this article for some basics, like the difference between a fused (fabric is glued together) and canvas (sewn together) front suit.
Once you have an idea of how to tell a nice suit apart from a cheaper one, you can try checking thrift stores, discount stores like Marshall's, and high end outlets like Sak's Off Fifth and Neiman Marcus Last Call. You have to wade through a lot of crap, but you can find great bargains. Websites like bluefly.com and yoox.com also have some good options, and a friendly return policy to boot in case you don't like what you get.
Ebay is the best place to get deals, BUT, you need to know a lot since you don't have the luxury of feeling or trying on the suit. Ask the sellers lots of questions to ensure he is reputable (a bad seller will avoid questions or refuse to answer some), and be 100% sure the measurements will fit well on you, because there's usually no return policy. Unfortunately, a lot of ebay shopping is about knowing brand names and being able to sort the good from the bad, but thats the risk. Definitely don't go this route unless you have this stuff down or have someone experienced in your corner (there are some helpful forums I could refer you to if you cared). I got my tuxes off ebay; they were great deals. However, I also see lots of men paying inflated prices for low-quality crap. Caveat Emptor.