When I first starting working at The Lance (over twenty years ago now, aack!), they had just burned $6,000 — in 1989 dollars, mind you — on a professional desktop-publishing setup. This included a 1200 dpi laser printer, very high quality for the time. One thing I remember about this printer is that there was no such thing as a toner cartridge for it. Instead, there was a simple hopper: you lifted a lid, scooped up some raw toner with a plastic scoop, dropped it in. Repeat until it was full and then you closed the lid. Easy.
Too easy! How are they supposed to gouge you when you can buy bulk toner and just scoop it in like coffee grounds? We all know that printer ink/toner is a racket… Now here’s video proof.
Lorne Beaton's Blog 
My printer ran out of ink maybe three months after I installed them, and they were only used sporadically. A year later I can still print the occasional page using only the black cartridge. My printer is a Dell, and I’m only supposed to buy from the company, but do you know if I bought them at, say, Wal-Mart, would they be compatible?
You can only use Dell cartridges, unless someone else is making ones that are compatible. Where you buy them from isn’t really important. There are some businesses out there that will refill the catridges for you instead of you having to replace them. I would check Kijiji for your area and search for ‘printer ink’.