I need help finding the right rubber gasket for my pool heater.

The unions that connect the in and out piping to my water heater have flat rubber gaskets that they don't have at any of my local pool supply stores. I did find an O ring that is working for now but it is a little bit thinner and I had to slightly stretch it to fit. I did find a perfect size O ring but it was slightly too thick to push into the groove that the original went into. The original is broken and a bit distorted but it is approximately 70mm in diameter while the gasket width is 2.7mm and its thickness is 2.2mm. Photos attached. Assistance will be appreciated, thanks. Attachment 971Attachment 970Attachment 969