Goodwill, garage sales, craigslist. I get luck with CL - had someone with a take all ad, ended up being about 200+ jars with lids and rings, extra lids, and they wanted $50. I'm pretty set with jars now
Eventually I'll have to order lids, but I'll do a bulk buy and get a few hundred.
Cari, I have gotten great deals on the Mason Jars at the thrift shops. I have seen them on Craigslist and Freecycle. Sometimes it is just not worth it to drive to get a few jars. Wal-Mart marks them down to get rid of them at the end of the season. I usually will grab the lids at that time. I have seen the case of brand new jars at Wal-Mart for around 3-4 bucks so not bad for new. You just have to be on the lookout for them. A neighbor was moving and left unused cases out on the street so people usually have the intention to can and then don't but they bought the supplies.
Bertoli spaghetti sauce comes in those jars. I clean and save all of them - AND it is the only botlled sauce that is not full of high fructose corn syrup!!