Auctions and estate sales are a great source for bulk purchases. You can also make the rounds at Goodwiil type stores and see what's been dropped off, but their prices are getting high also. I focus mostly on classical LPs, which are not in high demand. Since they are classical, they were, for the most part, previously owned by adults and are in VG+ to NM- condition. I do get a lot of stuff I would never listen to mixed in, but it works out over the long run. For local auctions, check out auctionzip.com. Where I live this is a very good source.
78s are a fun to collect. You can usually find crates full of them at flea markets and such. They might be marked $1+/disc, but can be talked down quickly. If you've ever picked up a bunch of these you'll know why - very heavy. My kids though I was nuts over Christmas when I set up a mono system and threw on Bing's White Christmas....C