I just replaced mine w these:

http://www.amazon.com/Infinity-Refer...y+speakers+4x6

I had to cut up the brackets a bit to make them fit, but they fit the grill opening in the plastic....

You could always search the interweb for 4 inch round speakers, if you want a direct swap...these look ok:

http://www.amazon.com/Rockford-Fosga...speakers+4inch