I've used the prducts at tonerrefillkits.com for years without a problem. I take the old carts from the office we would toss out (as virtually nobody buys them anymore), so the drums are never more then a refill or two old, and just refill them.

They sell a kit for $13 that has a soldering iron with a piece of pipe on the end of it that heats up to cut the hole, a plug to plug the hole once you fill it, and then you just buy the toner.

If tossing out old carts once the drum is no good (usually five or six refills) make sure to open it up and get all the toner from the overflow area too, that's usually full after a few refills.