For the bus plotter I was using arrows that google provides:

These are semitransparent blue equilateral triangles, with one for every 3 degrees. They are in the files dir_0 through dir_117. The downside is that you can't see which way the arrow is pointing. Becaus the buses I'm representing move, it's not usually a problem, but I had request for better markers. I didn't find any, so I made some:

Get the whole set on github or tweak the python program I used to generate them if you want something different.

(Mine are slightly more transparent, which was originally a mistake, but I like them this way.)