Ebay all the time. I have a Brother DCP 7020 and I buy my toner
from there all the time. It's a few bucks cheaper than Amazon and
way cheaper than anywhere else I've seen. Make sure that you buy
OEM toner sealed in its Brother packaging, not remanufactured
generic toner. I got burned once doing this. Using generic will
void your warranty. I tend to search for newly listed Buy it Now's
because those tend to have the best pricing.
Also to note is that it's okay if it is expired. It still works
well as long as it is still sealed.