I think what you are looking for is something like the Gas Burner Control Module fitted to most gas storage water heaters, have a look at this link Anatomy of a Gas Water Heater (Tank type)I think you should be able to pick one up from most any plumbing supply or specialist hot water plumbing supply. The module includes the igniter, the pilot burner and the flame out sensor.
You would need to fit your solenoid after the module. A lot of the modules have a thermostat that rests against the tank or a probe into the tank; dont get that warm! Make sure that isnt anywhere warm and it wont shut the gas off leave that to your controller and the solenoid.
You can get standalone units, I have one here in a box that I am planning to fit to a 1000L Kettle, its BSP In/Out and cost about $400 (safety first) but I think if you are a bit creative a HWS system module will give you both the safety of the flame out protection and the control you are looking for.
You dont say where you are from in your sig, but Combustion Instrumentation here in Newcastle (office in Brisbane to I think) were very helpful.
Mark
I'm getting one of these: HONEYWELL GAS VALVE VR8200A 2132 24 VOLT From US patriot supply on ebay seems the cheapest even considering the freight. Only problem with it is the heat signal has to be 24v AC.
So you also need to buy a transformer.
They also stock those but I had to contact them for a price.