I have been looking at the balanced thermostat as the way to go on my cars.
The Prestone parts book shows a Gen 2, PN 775-195 and a Power Performance, PN 330-195 as the thermostats that they recommend for the MGB Prestone also has a 180 degree PN 330-180
Cobra Valley has a good write up on the balance thermostat and has one available.
Milodon has one also.
Scroll down to PERFORMANCE THERMOSTATS PN 016401
Summit has Mr. Gasket ones. I believe they are the Prestone ones repackaged.
EMP Stewart Components has them.