The manual for my tractor specs the tire pressure at about half of 240kpa on the tire sidewall, so about 15 psi.
If I wait a little while they will leak down to 15 psi, down from 30 psi that I filled them up too. I'll start hunting for 9.5-24 tubes. Forgot the front tire size. 6.00-15 I think.
I'll be happy to wait until these tires are old, before I replace them. This tractor spent it's whole life in the Arizona desert sun. 16 years old now.