The more common 5v VR is capable of accepting and input voltage range not one specific voltage. This one for example
https://www.sparkfun.com/products/107 has an acceptable input range of 8-20v. It of course it not the one used on the thrifty throttle, it is just the first one that showed up in a search. If you can find the part number of the one used in the Thrifty Throttle then you can check it's data sheet.
The other option is the diode trick. A silicon diode has a voltage drop of about .7v so string together 5 of them in series between the nominal 12v battery which is actually 12.6~12.7v when fully charged and you'll have a ~9v input to the Thrifty Throttle. If you are using it for testing that will have a significant load that will cause significant voltage sag on the battery then you may want to use only 4 diodes.