Hi,
Here’s a link to the [https://data2.manualslib.com/pdf3/71/7040/704000-canon/ae1.pdf?c34b9b3ff1674181fafec0cf122b4703|service manual] for the camera.
Scroll to p.20 to view the procedure to remove and test the mirror assembly.
Couldn’t find the specific replacement part that you’re after but here’s a [https://www.ebay.ie/csc/Replacement-Parts-Tools/182074/i.html?Brand=Canon&_nkw=canon+ae1+program&_sop=3|link] to a page of suppliers of various parts for the camera. Some say “other parts available please ask” or ‘other parts available”, so you may have to do this to get the required part.