This is what I bought, and I've been pleased with it. You have to pour hot water over the gauge each time you read the gauge, to make it read accurately.
https://www.amazon.com/21st-Century-.../dp/B0001UZRBG
It's my understanding that the gauges that get screwed to the top of the tank, are inaccurate, in that they show lots of pressure until the tank is almost empty, and then suddenly the pressure gauge drops precipitously.