paintball bb's
 

Where to get them?

Unless you are using some craptastic Wal-Mart/Canadian Tire bottom of the barrel gun, avoid them.

I want to use them on an m203. since it doesnt have any electronic parts to worry about

There are many online retailers where you can source 6mm paintballs.

If you're planning to game, other players aren't gonna be OK with you using them.

It's actually for a paintball gun, The m203 takes both Paintball shells and Airsoft Shells. But the Airsoft shell has a larger spread better for taking people out. The Paintball shell only shoots 4 paintball out.

Flying Colors is a brand of paint BBs if that helps your search.

You joined just to ask this?

There is a real good chance that an airsoft 203 shell isn't going to fire the paintball pellets with enough force to break them on contact with the target at a reasonable distance. There is also a good chance that the ones that do break are going to break in the barrel/grenade when you fire the shell.

You are probably a lot better off using the one that fires the four actual paintball rounds.