It sounds like you have plenty of room for the 11R22.5 rear tires. I measured a 8.25x20 and a 11R22.5, both fully inflated on rims, and found them to be at a height of 37 and 41 inches respectively...just an average indicator.
As for tire and rim sources try some local trucking cos and see if they have some in their boneyard. We have sold most of ours and got on average $35 apiece for good open center 22.5 rims. Another source are dry storage trailer rentals which lease out old dry vans no longer suitable for over the road use. Truck salvage yards are yet another source. As for tires the local large truck tire service centers may have some good used take offs that long distance trucking cos cannot use but a local trucker like yourself can.
James
