https://www.amazon.com/Mongoose-MG78457-6-Fat-Tire-Tube/dp/B01CGDVXL8/ref=mp_s_a_1_3?crid=1W7D50VEGIDPV&dchild=1&keywords=20x4+bike+tube&qid=1630601647&sprefix=20x4&sr=8-3
I was told by super73 support that these tubes will fit the RX wheels. https://www.amazon.com/dp/B01CGDVXL8