The Mamiya 500/1000DTL camera bodies use a modern LR44/A76/S76 battery, always have for some reason, despite being made in the mercury battery era.
Since your in Eastern Europe maybe you can get a Kiev 88 pretty cheap
I also have been meaning to sell my Canon AE-1P, I much prefer my F90X/N90S and Yashica Lynx 14 for 35mm duties.
I think I have about $500 US in my RB67 kit, two lenses, two backs, a Metz 45CT flash, Metered prism finder, waist level finder, hard case, a nice heavy duty Vivitar tripod........