I got my bbt therm from Drugstore.com. Cheap and they send it to you.
I would advise that you get a bbt though. You can do it with a regular one - but they don't register changes quite as well. (I though you could use anything until I was a FF guide, and saw the differences between a chart using a regular therm and a bbt.)
Here is a link to the one I use (and have been using for two years), it had a light, keeps your last temp. BD Basal Therm