Hrm, free graphics are in pitifully short supply on the Internet! For some reason artists don't have as much of a community spirit as programmers...
The famous Ari Feldman
library doesn't have any isometric tiles yet, unfortunately.
You could try looking for leads from http://www.isometrix.org/
- I just found it from a Google search and it looks like it should have a few good links. It's also got a section on math relating to isometric engines, which may be of use to you!
If you want to try creating some art yourself, you could do a lot worse than looking at http://www.zoggles.co.uk/
- an site by an excellent pixel artist who specialises in isometric perspectives. He has written a good number of tutorials and hosts a lot more. There's some fantastic bits of art in there.
Anyone know of any good, free isometric tile sets?