Sorry. Everyone seems to be hung over today. Usually if you post something about Thomastic flats, you get replies immediately. The ones you want are the 34" scale strings, the JF344 set. That fits standard Fender J and P basses, and almost all other 34" scale basses.
You'll love them. I have them on six basses. They take a few days to play in properly and the sound gets richer as they get played. Then they stay like that for years. Over 6 years so far on my Fender P, and they still sound great, and up to 5 years on some of the others. You may have to do a slight setup of the truss rod and action if you're changing from typical high tension round wounds, but nothing drastic.
Edit-where to get them? They can be hard to find in your average music store. Lots of places on line. Carol Kaye sells them on her website and ships fast.
www.carolkaye.com Also Steve Barr at the Dudepit has them. I think Juststrings does too.
I have a source at a store in Toronto, but I've ordered from Carol Kaye also and got them quick.