Hmmm... I agree with Gamer. Even if you could provide a little sample of the type of 'graphic style' that you're looking for, it would help a lot.
I found these...
hirefreelanceartist *dot* com/free-tower-defense-graphics
Sorry but I can't post links until I reach 500 rep so I had to place it like that.
Even if they're not what you're looking for they will work wonders as place holders for you to get your project started.