I love these guys. For 8 bucks plus change you get 36 of them. They are about the right size for a standard gaming mat. They are good for generic enemies, but also have enough variety to differentiate them from each other. Also, half are black and half are grey, which is useful. I like to use one color for mooks and the other color for either allies or special enemies.

For larger enemies my friend use a bunch of the laminate tile samples that she had gotten from Home Depot and glued pictures to them. I'd imagine the same thing could easily be done with cardstock or cardboard.

I do have some actual mini, but we reserve them for player characters.