Yes, I would definitely get an actual BBT thermometer. I originally purchased an Up & Up one from Target, but it died on me within two weeks. Now I have a Mabis from Walmart. I would still temp even with OPKs because you can get a positive OPK without having actually O'd. Someone correct me if I'm wrong, please, but I believe OPKs only show that you are gearing up to O and the temps actually confirm it.
ETA: Also, don't get discouraged after three months. I know that's easier said than done. It can take up to a year for a healthy couple to conceive. Just keep temping and using OPKs to get the best timing possible. Hope you get your BFP soon!
You should also check out the newbie link at the top of the TTGP board.