Rubber or plastic? I don't know of any rubber bullets on the civilian market, but Speer makes plastic bullets and plastic cases that use only a primer and can be reloaded without a tool. They work well, but BE CAREFUL as they have more power than you might think.
I once tried them by taping a target to a cardboard box, which I then set on an armchair. The bullet went through the box and completely through the back of the chair. Ooooppps! (The chair was an old one, used for relaxing in a shed; had it been a new one I would still be hearing about it.)
Jim