I happened to cruise by the Doran site the other day and I saw that they have a new line of gadgets and the TPMS I brought from them was on clearance
I was a bit alarmed by this because I expect to have to replace the sensors at some point. So, I contacted them by email and they got right back. If you have a Pressure Pro system, the new owner is Pressure Pro Systems (wonder how they got that name?) and their website is
http://www.advantagepressurepro.com/. It looks like they have the replacements that I will be looking for in the (hopefully) distant future.