Help finding Canon F-1 Diopter

[dazedgonebye]

I'm in need of a +1 or +1.5 diopter of the Canon "R" or round type to fit my F-1.

Does anyone know where to find one or know if some other maker's diopter will fit the F-1?

 

[Spavinaw]

I found my Canon diopters both round and square using the following method: eBay plus diligence plus patience.