I get them at Target & here in NY, it's 7 for just under $15. They're the generic CBE brand. They've always worked well for me. I also have used the strips when one of the girls on another board sent them to me after she got pg. Those were fine too.
If you have a rough idea of when you O, you can use the 2 remaining OPKs just before you think you'll O & you shouldn't need another box. For me, I usually know about when I'll O every cycle so I don't test & test. I was just super anxious this month

But if you aren't sure, better to be safe than sorry.. just buy more.