The pressure gauge I got from Amazon for $9, I think it's meant for those under sink water filter systems. The level sensors were $24 each on ebay, while the gauges where $16. Whole setup cost me about $100 for everything including the fittings. Gauges seem accurate also. Let's hope they last.

Sensors used for the 75 gallon tanks: https://www.ebay.com/itm/163957390803

Gauges: https://www.ebay.com/itm/362427662272 https://www.amazon.com/dp/B01KU6C4E8?ref=ppx_pop_mob_ap_share