Play Keyquest in a 5 key game... Win. (With a two player game, you and someone else) Easiest way to win a paintbrush. It might not be the one you want but there is a chance you might get it. If not, you could sell the other paint brush to earn enough neopoints for the Baby Paint Brush.
It will take time, you don't always get a paintbrush... But you do get other valuable items most of the time. I haven't played in a while but that seems to be one of the easiest ways to get paintbrushes or valuable items for me.
Even if you don't win, you can still get some nice items since you would win a silver key. Winners get gold. If you play with lesser keys, you tend to get lesser prizes and metal keys.