My roommate and my wife have always loved hitting the thrift stores such as Goodwill - mostly for non-clothing stuff, but we've found me some nice things there that I really like. The obvious downside here is that quality of the items is going to fluctuate greatly and with my size I don't tend to see as much as they do - although that varies by thrift store around here.
That being said, my favorite spot is DEB at the mall. I don't have the sizing issues that I would at a thrift shop as they carry plenty of clothes in my size. If I had to guess, probably over half my wardrobe has come from there and all on clearance over the last year. Normally if we get by there I'll try and grab a thing or two on clearance - usually only things under $10 depending upon what it is and how far it's been marked down. However, our last visit (which was by accident as we were dropping my wife's ring off to be cleaned) they were having a huge clearance and we walked out of there with 23 items - about 2/3 of them mine - for $98.xx (about $90 before sales tax) so about $4/item. At that point it's priced in the thrift shop range and hard for me to pass up. I had a *tiny* bit of buyer's remorse as I hadn't really budgeted for that expense - as I said I normally confine my shopping to less than that for any given outing; but we made it up elsewhere so it all worked out.
As far as eBay goes, I've gotten a few items from there, but I normally prefer to avoid that since if I hit the clearance sales like the one above I can usually get by cheaper plus I get to try it on then and there and make sure it fits. However, there are bargains to be had on Ebay and there are a lot of neat things that I don't necessarily run across at the mall so I do keep an eye out.