Hi Sandra! I use a BBT that I purchased at Walmart. It's white with a pink cap on one end. I love it. It saves the last recorded temperature so you can take your temp early in the morning and just go right back to sleep because you can just turn it on later and see what your temp was.
I would avoid buying your BBT at Walgreens though. A few weeks ago there were a whole slew of women who bought those and they all noticed they were getting the same temp day after day, so that didn't sound like a good one to buy. And the one at Walmart is less than $9.