What kind?
I get my Epson cartridges (T033 series) from Staples' Inkdrop program. After you sign up, they send you envelopes. You pop the old one in the envelope and send it to them. They send you another in a day or two. Free shipping both ways.
Of course you have to keep one of each color in stock, but no one stocks this kind in stores any more, so you really have to get them this way. I'm afraid when the printer finally dies I'll be stuck with six new cartridges; I suppose I'll have to sell them on eBay then.
I too gave up on refill cartridges long ago. At least 50% of them failed or gave lousy results
, so there was really no saving.