Where I have had reasonable success is trawling through stores like
Goodwill, Salvation Army, ARC, etc. to find toner cartridges for my
HP LJ III and HP LJ 4L. They are usually still-sealed packages ("NOS")
and the prices are usually $5-$10.

I used to have a couple of LJ IIIs, but now run a 4. I have heard a
story, which agreed with my experience, that LJ III toner cartridges
were deliberately designed with rubber wipers that distorted after a
few years, to discourage their refilling by third parties. I know I
opened two still-sealed cartridges for my III and found they produced
vertical streaks.

In any case the LJ III was only 300 dpi while the 4 is 600, so I
would never go back. Combining parts from two machines, for a total
cost of $35, got me a nice reliable printer.

Alan