The tubes are definitely scarce. When I replaced my tube last year, I had so much trouble finding it I bought two - I carry the spare with me.
My shop scrapped my carcass
- I wanted to send it to Desser for a retread.
I've seriously considering switching back to the "normal" tire size; but it's non-trivial. Everything but the gear leg gets swapped. Bearings, wheels, brake assemblies...it all goes.
Here are three sources for tires, all near $200 or less.
https://www.aircraftsupply.com/ab2a6.html
https://shop.desser.com/18x5.5-8-ply...detail/323311/
https://www.univair.com/tires-wheels...air-hawk-tire/