I don't know of a source but if you find a stock of them I know several owners that would be interested.
I personally only found one new old stock exact match to the original in my car since I started looking in the mid 1980s. I have found several ones made in different years, with different color plastic over-moldings, and many with a different temperature actuation points. Most service parts have a black plastic molding. I have found some with a natural uncolored plastic molding. During the Cobra production time period there was at least two versions.
Most owners of original cars do without (adapt to manual control with a toggle switch somewhere convenient) or adapt a modern switch of their favorite type and temperature trip point.
I visited a shop in November that has been servicing original Cobras the past few years for owners and they have yet to find a source of exact match new old stock switches either.