If you mean the vertical column with a needle valve on the bottom kind of flow meter, then pressure isn't very critical since you will be using the needle valve to set the flow so anything within the allowable range for the flowmeter - say 20 to 200 - will work.
So any regulator, or even a needle valve capable of handling full tank pressure, will do the job. However, the problem is hooking the regulator or reducer to the SCUBA valve, and the cheapest/easiest way to do this is usually to use an old SCUBA 1st. I've had good luck attaching the flowmeter directly to the 1st, using a brass adaptor made from a hardware store brass fitting - I think there's a picture of one such rig in the Gallery section of the Airspeed Press website.
You should also know that a lot of diver's aren't using regulators and flowmeters anymore, but simple samplers that fit on the tank valve, and using the tank valve to control flow. Crude, but usually works fine.
I've attached such flowmeters to old SCUBA 1st stages, I think there's a picture of one in the Gallery section of the Airspeed Press webpage.
