Do a true diameter check on the tire. Hold the wheel perpendicular to the ground. Hold a level on top of the tire and get level. Measure from the bottom of the level to the ground; that'll be your true diameter.
For example my 26" Kendra 838 tires true diameter is 26.25"
Sorry to repost late in this thread, but I ran across this link for various sprocket sizes that looked like a good price ($10.99-$15.99 ) with free shipping: