You did not say what your flowrate is, or your line size.
Almost any filter-regulator will control to that downstream range: Fisher 67FR, Masoneilan 80-4, Leslie AS-1, and variations from Bellofram, Norgren, and Parker. Typically these are 1/4 NPT, with aluminum housings and a port into which you can screw a pressure gage. HOWEVER most airsets such as the ones mentioned above are rated to 250 psi. You can even buy airsets such as these at home stores, used to regulate air to spray-painting equipment.
If you MUST have a 300 psi inlet, consider the regulators that are used on Nitrogen bottles. These are rated for over 1000 PSI. CHeck your local welding shop.