Metal prices have been shooting up. Frustrating. I can't think of any reason you would have problems with A36. I'm not an expert with different metals though. I would avoid any cold worked, that tends to leave stress in the metal that can cause the warping you mentioned. You might try looking for "stressproof", 1150? Not positive on the number. Don't go too nuts on material, it's a fancy spacer, it doesn't need to be anything too crazy.
Try watching ebay. There are a few sellers there that often have random drops. Sometimes you can get lucky with a piece like that. Someone here might have a piece they are willing to part with as well. I don't have anything that size.
You might try asking the foundry for this...
https://www.patreon.com/posts/solid-toolpost-108954223 It looks like a nice casting, and you would have to flatten the top and bottom, but I expect you could adjust it to match your lathe. It might be more expensive, I don't know. Just another possibility.