You can buy a BBT at Walmart, Target, CVS....They aren't too expensive either. Using OPKs would help, but temping is best for helping you figure out what is going on with your cycle.
I think temping and OPKs are both really useful. Opk's can help with timing, and can help FF determine your ovulation date when your temp rise isn't completely clear.
OPKs aren't for everyone, but I do think they can be incredibly helpful.