They are in my Contax RTS and Bronica SQ, my SQAi has their SR44 cell in. Just make sure you don't buy the alkaline version (about 3 quid) that would work fine in a 'car' fob but the voltage will fall off in use so not good for cameras, though even the alkaine version would get you through a weekend shoot if you are in a hole. (BTW I don't work for Maplin, but I can walk to my local store in 10 minutes)
EDIT: As regards 'off brand' I have bought well known brands that must have been in the shop for years because they would not even work the battery check on the RTS. These days I would rather buy off brand from a busy shop rather than premium brand old stock.