flattracker makes a good point, one I was not actually aware of. However, I'm pretty sure such parts will likely not be readily available at an O'Reilly's or AutoZone...
some of the opreilly and advance auto as well as napa stores have commercial parts.. one of the things the parts stores are getting better at is showing what store has what inventory of parts real-time.. NAPA stores listed as machine shop locations almost always stock some commercial truck parts.. Oreilly is a toss up however going to the www and looking for parts will show if a store has it in stock or not.. and of course you can call and they have access to their whole "district" on computer normally..
thus far Napa and oreilly and advance have been able to look up "International" or "freightliner" etc vs just the consumer brands.. so i think they are trying to be more broad..
I have the same problem on my 30ft International with a wheelchair lift on the passenger side. Mine leans way more 4.5 inches on drivers side to 2 inches on passenger side. I was missing the wear pad on that side. I replaced it and went from 1 inch to the 2 inch height from tire to body. The wear pad goes on top of the front part of the leaf spring. I had to jack up the frame and loosen the axle U bolts in order to get it to sandwich between the leaf spring and the hanger. I could find very little about the leaning problem. Now since reading here I’m going to find that check valve and replace it.