well i think the stock tires are 225 55R 16, you can find that on the side of the tire... first off the first numbers are the tire width in millimeters, the second numbers the 55 is the aspect ratio... this means that the sidewall height of the tire is 55% the width of the tire the 225 number... the R means radial and the 16 is the rim size...
try
www.tirerack.com they have decent prices...
Steve