I doubt you can get Frontline at a feed store, unless it's a really good one. Try your local vet, or get online and order it. Also, you might try dusting his house with Diatomaceous Earth, also available online and in better farm stores. Even some health food stores have it.
The frontline...
They say frontline spray works really well. I hear it's kinda expensive, but worth it.
Have you totally disassembled his condo and washed it out? Those little buggers hide in all the cracks.