New Old Stock sleeve bellows for the XKs, and Early Series 1 E Type, are on ebay occasionally, look for “bellows sleeve thermostat”, and search “Smiths” (will be 85025/xx, or the AC TF1 or TF3, etc…takes some tries to find em…and be sure to search ebay UK. You want 68C, 72C, 73C, or 80C…but prob not the TF2 or 4 that are 86C. Must have a sleeve to close the bypass, …so not the bare bellows types, not the TC.
around $40-50 USD. These are for early 3.4 S1 that use a 4lb radiator cap. Do not use a higher lb pressure cap…this bellows type of thermostat will be damaged, possibly even at 7lb. The later type C3731/1 thermostat currently sold by the usual vendors with the sleeve at the top, can be used with the 7lb cap.) By the way…the 3lb diff, 4 to 7, provides only a few degrees of boil point: anti-freeze 70-30 would do much more.
Nick