Wellhouse - Pressure Gauge
Winters PEM208LF PEM-LF SERIES (LEAD-FREE) 2" 0/100 PSI/KPA 1/8" NPT LM
https://smile.amazon.com/gp/product/B00GD8RC72/ref=s9_acsd_al_bw_c_x_2_w $11.15
This is lead free 1 - 100 psi. It is the best one to standardize on throughout.
You will need an adapter from 3/8" NPT to 1/2" MNPT.
1/2" x 1/8" MIP x FIP Brass Bushing (Lead Free)
http://www.supplyhouse.com/Everflow-BRBU0120-NL-1-2-x-1-8-MIP-x-FIP-Brass-Bushing-Lead-Free $2.23
Then fit an adapter from 1/2" FNPT to 1/2" PEX...
http://www.supplyhouse.com/Bluefin-PXF050-1-2-PEX-x-1-2-NPT-Brass-Female-Adapter-Lead-Free $1.60
For any other brand you buy, make sure it is rated for potable water and lead free.