1st 'd replace the thermostat you're using to insure it's not worn out. Are you sure you don't have a heating core issue too ?
I'm sure you will only find a standard temperature T-stat listed, probably a 192°F (88°C). You may find a colder one listed, but that's not what you are after.
It is possible to build one that is a higher temperature by assembling components of 2 units. I have done this to create a 195°, 198°, 203° and a 205°(high flow) thermostats for my 300Tdi. I had zero temperature gain by blocking off the radiator. A higher temperature unit will also make the system pressure also increase, so you may find leaks that you were not aware existed. Even when I run the 203° in the coldest part of the winter, my radiator cap does not blow off which tells me that I'm still below the system maximum pressure. I have not run the 205° high flow unit yet (40% greater flow) because higher flow will actually result in a lower temperature because it allows the water in the bock/head to exit faster therefore not absorbing as much heat to be dissipated in the radiator. http://www.defendersource.com/forum/...tdi+thermostat
If you decide to do this, I'd recommend that you 1st install a VDO Temperature gauge by their instructions to be sure of it's accuracy. Maybe get yourself an IR gun (+/-2°MAX) & measure the temp at the sender to be sure of accuracy seen at the gauge. http://www.defendersource.com/forum/...ir+thermometer
I use MotoRad thermostats to make mine. Here's their catalog link. They are found commonly on eBay.
You will need to maintain the D1 diameter and the approximate over all length (AOL). The wax pellet (copper thing in the middle) does come out and you can carefully unwind the spring out of the pressed together housing, replace the pellet and screw the spring back in.
Here's a link to help you in this process. http://forums.tdiclub.com/showthread...=306799&page=4