I used to buy all the OPKs at the store. Usually Walmart as I found they were the cheapest where I live in Canada.
Also bought a fertility monitor on EBay.
Hope this helps..Good luck!!
early-pregnancy-tests . com I ordered from them and they're not too expensive and you usually get free shipping and it comes with in a week. I ordered OPKs, HPTs, and Preseed all for 32.00.
I got my OPK's at the store and they were only about $6.00. They were the Answer brand. I used them for the first time last month and got my BFP so I would say they worked well for me.