All the new thermostats I have found that are "millivolt" say they are for 750 millivolt and the existing one I am trying to replace says it is 700.

Can I install a 750 millivolt thermostat on this wall furnace?

If not, please suggest where I can get a 700 MV thermostat.

Any 750 millivolt stat will work just fine as long as the thermopile/powerpile is still in good working order.

Thanks!
The current thermostat works, but is old and case is broken, so I assume we should not have any problem as long as we get the correct type.

Yep, should be fine.

Installed a honeywell heat only 750 millivolt and it works fine.

Hey, glad to hear it. Thanks for the update.