Where can I buy a rebuilt fan motor for a Durcane AC model # AC10B30-A

You probably are not going to find a rebuilt motor.

I agree with Grady. However, you may be able to find an electrical shop or hvac shop that will rebuild the old motor for you.

Assume you mean Ducane. Google Ducane Motors. Lots shown but since you didn't give us the motor number, not sure if one fits yours.