I buy most of them online at
http://www.early-pregnancy-tests.com/ I really can't complain about their prices. I don't buy the mid-stream tests. I get 25 HPT's for $20 with no S&H. OPK's, since I just started and was unsure of how many I would need I bought 50 for $30.
If I run out I either go to CVS or Target. Yesterday I bought some HPT's in the Dollar Store.