Ugh... do NOT buy them at Michaels or other craft stores - I've seen them for $1 each or more. You can almost always find them at a hardware store (like your local Ace, not Home Depot) and often WalMart for $5-6/doz. Thrift stores and garage sales are also great sources. Grocery stores around here carry them year-round, as well, along with lids and rings (if you need them).
(the canning queen)