Coming to you from the G.O.A.T. of tube manufacturers: CST! These are what you need for your 18 inch freestyle BMX bike!
Schrader valve tubes (or car valve) are the most popular tube type for freestyle BMX bikes and a number of BMX race bikes. With a versatile car valve, you can pump up your tyre at any servo around the country. Find the right sized CST tube for your 20 inch right here, and just don't just grab one, you can never have enough spare tubes in the tool bag.
PRO TIP: Tubes stretch! Tubes will just fill your tyre; so, 2.125 (or even smaller, honestly) is more than enough rubber to work in anything up to your 2.5 inch tyres (or fatter).
