Ask HN: game devs, how do you mock-up your graphics?

I am currently working on a multi-player cross-platform game which can best be described as a mix of Rome: Total War and Jagged Alliance 2 with a lot of crafting and scripting going on.

Anyway, my conundrum is that I need some placeholder graphics for my game until I can find a graphic artist to work with. It's not really enough to just draw rectangles and circles, I need sprites to represent soldiers, workers, houses, stuff like that.

Is there a place where I can find free sprites (it's in 2D) for my game? How do other programmers typically do this?