Ovulation tests

I'm looking for advice on which ovulation tests to buy. My cycles are not regular so I will need approx 15 per cycle. I would like some that are reasonably priced but also reliable. We have been trying to conceive for 3 months now. I purchased a brand from eBay and never got a positive or even close to a positive. I don't know if it's me or the tests. Has anyone had this problem before? Thanks in advance!