Frankly I get mine from Amazon.
I've had great luck with the Unibene Cross-style refills. They are inexpensive and seem to be a significant step up in writing experience from the refills included with, e.g., slimline kits.
The gel Parker-style refills from the same Unibene brand are meh. I won't buy them again. I'll just spend a bit more on Monteverde ones.
I just discovered the Easy Flow 9000 from Schmidt, which I've ordered from Lanier Pens (also via Amazon). They are hands-down the best ballpoint refill I've ever tried. They're Parker-style and blow every other ballpoint out of the water. I think I might even like them more than gel.
For rollerballs, the 888s that usually come with pen kits are good. I prefer the metal 5888s with a medium tip though. Once again, Lanier Pens via Amazon.
If Schmidt made an Easy Flow 9000 type refill in the Cross-style format, I'd buy 100 of them immediately.