We use http://www.rushind.com/index.html for our fans, ring them and tell them the shaft size of the motor and the diameter of the cowl. May be usefull to let them know if you need a flat back or a concave back type of fan.
First place I'd try would be the local scrapyards. They'll usually have some motors knocking about, and will charge practically naff all for them. You'll likely find something to match yours.
My local scrap yard has several skips full of motors.
So that's where I always headed for stuff like that[or the pub opposite my local re-winders at lunch time]. Or fit an electric fan, my mill spindle motor runs[if I had the drive] from 0 rpm up so it had an electric fan on it.