There doesn't seem to be many places to buy the HW90 gauge in the USA, Airguns of Arizona is the only place I've seen so far and it's out of stock. I'm not certain on the fittings.
Buy a guage wherever you want cheap it's the correct threaded adaptors that a good luck chuck thing..

One guy had a 500 psi shock pump that had a Guage on it so he just added the rams adaptor to the house and was ( or so it seemed ) good to go ..
 
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