Thanks everyone for the help!
I've just tried Alan Gordon but sadly no luck.
I did think about using a double 8 spool but all the ones I could find were 25ft so I would probably end up with the same trouble trying to find a 50ft one of those tool.
After much searching the only one I've managed to find so far is this old film on eBay which I missed by a few days. If it's anything to go by it looks like 50ft spools are very old. I'm beginning to think the only way to get some might be to either have some 100ft spools cut down or find a bunch of old 50ft 16mm cameras that have spools in them.