The rrp on the Jamerson set is something ridiculous like £50 but I got mine from here (scroll down just past the steinberger for the deep talkin sets). The only other flatwound type strings I've tried were Fender tapewounds that came with the bass but I'd definitely recommened La Bella and proper flatwounds (tapewounds are sort of bumpy although they aren't rough like roundwounds).
Here's a pic comparing the La Bella to the Fender that shows the difference in smoothness.

If you're thinking about getting a set but think the price is a bit high, from what I gather you pretty much never need to change them. Jamerson and Donald Duck Dunn both used heavy La Bella flats and kept the same set on for their careers (changing individual strings if any broke I think).