Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Would get something like this, which is a nice easily read size for a gauge.
It is 0-30psi, but that would also keep you covered, when running your engines on full pressure (max standard safety valve setting).



There is quite a few at very fair prices, if you trawl through Ebay:

0-30psi:

0-15psi:

Do go for an American or European made gauge though, as the Chinese may be tempting with their low prices, but precision does follow the price.