Not to be rude, but Photoshop 6.0 is hardly ever used. Ever. You will have an impossible time trying to find any sort of brushes, I did some searching, and all I found was people asking the same question you are. Your not going to find what you are able to find in sigs that you see around.
The only decent brush site I found was
this, and it doesn't contain any grunge/abstract brushes. Just ones for touch-ups (which imo, is the only way brushes should be used), so, good luck...
Honestly, you should either get a better version in another way. The easiest way to get a good,
legal version is to save up 90-100 USD and get the latest version of Photoshop Elements. Which has an insane amount of features for being 1/6 of the actual price of CS2/CS3. It's a very good deal. Also, it's a good deal bit better than Photoshop 6.0/7.0, and it has the ability to read all brushes. Maybe it's just me, but any brush I download works with my good ol' PSE3.