Need new whole house fan motor

The motor of our whole house fan seized last season after having run for hours. It just hummed. Days later, it would run again only to stop after several hours. Climbed into the attic and removed the motor. The shaft spins freely. Maybe if I run the motor for several hours, the motor will stop running. But I plan to replace the motor as we have had many years of good use from this whole house fan. The problem is I cannot find an exact replacement. The motor is a two speed, 1/4 HP, 900 rpm, 120 volt unit. Not sure what the low speed RPM is. The 24 inch fan is mounted directly onto the shaft. Any idea where I can get a suitable replacement motor?