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).

https://www.ebay.com/itm/Ametek-P1535-110793-Usg-Pressure-Gauge-0-30psi-2-1-2in-1-4in-Npt-NEW/143934465269

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

0-30psi: https://www.ebay.com/sch/i.html?_from=R40&_nkw=pressure+gauge+0-30+psi&_sacat=0&LH_TitleDesc=0&_sop=15

0-15psi: https://www.ebay.com/sch/i.html?_nkw=pressure+gauge+0-15+psi&_sop=15

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.