I've bought thousands of rounds of "once-fired" brass from Ebay sellers. It's a great source for major quantities. About $50 will routinely bring 1000 rounds of .45 ACP, and $20 generally gets a thousand 9mmP or .40 S&W. The search words I use are "reloading brass", which also brings up new brass along with the used stuff.
Obviously, we have to take at least a small leap of faith when the seller says "once-fired", but I have found that by buying from people that have several hundred (or more) positive "feedback" comments, there's not much risk.
All my transactions to date have left me quite satisfied, and I'll continue to buy from such sources.
For an example of one sale I avoided, an Ebay ad was placed to sell 41 rounds of "used" .416 Rigby brass. What in blazes is "used"???? Once-fired, or ten times, or worn out? No thanks, pard.