In case you have not yet found the switches, look on Arrowhead Electric's web site:www.carbonbrush.com
The switches are from the 20TC series of Klixon circuit breaker switches. The number after the dash indicates the amperage rating of the switch.
Thus, my 10-amp pitot heat switches carry the part number 20TC4-AK-10.
I had the same problem that you did. The cover broke, but the underlying switch still worked. Sadly, they don't sell just the cover, so I had to buy the whole switch. The good news is that you can buy directly from them. The bad news is that mine was $275.00 plus shipping.
Hope this helps.