Does anyone know where I can get a new master cylinder reservoir seal from?

Mine is from a sierra with a float switch in the cap and it has a kink in it which lets it leak.

It looks a lot like this: (image lifted from tinternet)

Either... try the scrap yard as I suspect the seals for the res' are the same on many master cylinders.
Or... master cylinder seal kits come with the res seals. Try your local factor or somewhere like BrakesInternational.

adrian

Good idea I had not thought about it being in a seal kit - D'oh!