I get my toner refills from
http://www.refilltoner.com/ . First time you get a kit with whatever is necessary for your printer. So maybe a screwdriver, funnel, gloves etc.
You basically open the cartridge and pour the toner into the cartridge. Bung it back together and get printing again.
You can refill 2-3 times with no drop in quality and maybe more if you can accept a few dots here and there as the drum deteriorates.
I bought a Samsung ML-2240 from Argos because it had a kit available on that site and I read that with a paperclip and a steady hand I could reset the printer to 0 copies printed and continue to use the starter cartridge.
I haven't kept up with the latest models but I will have a quick look for the 2240s replacement and see if it has the same workaround.
The cheapest one Argos has killer chips designed by Samsung to prevent refilling, the refill sites sell you these chips but Samsung have firmware updates to fix that too.
Prior to the Samsung I was using an Oki machine which was much simpler to refill. Was too expensive for me to run though and I understand later models also use chips. Even with this machine I had to go into the maintenance menu and reset counters as each consumable had q limited copy life set in the factory, which was waaaay lower than they were capable of doing.
Just have a look at some printers and then search for refilling advice. I seem to remember fixyourprinter forum being useful too.