I recently got this exact bike, but in even worse condition. My LBS had the right size tires. I don't think they're hard to find. I believe the size was stamped on the spoke-side of the rims. They were a little tricky to put on, so be patient.

The biggest issue with mine is that the shifter is gone (though the cable is still there). Also, one of the struts that holds the fender is gone. Does anyone have any suggestions?