Any resin casters offer a variety of these (Wheel backs?)

[Monty]

Not sure what they're usually called, but I've occasionally heard them called wheel backs (see P/N 19 in step 6).  Hoping to find a variety of depths.  Multi-packs in styrene would work too.  

[Can-Con]

Modelhaus tires does some resin wheel backs Monty Don't know it they have exactly what you're looking for but here's a link,,,

https://www.modelhaustires.com/

[bobss396]

I always save up unused ones or find them at model shows. They come in handy.

[stavanzer]

Monty, try Ron Coon Resins. He carries some. Look at his wheel listings.

I know I have dozens of them, collected over the years.

And here is an eBay listing for some. They are high priced but he is trustworthy to deal with.

Remember, AMT & MPC wheels interchange (mostly) but Monogram are 1/24 so a bit bigger.

https://www.ebay.com/itm/166374467321?itmmeta=01K1ZTQBKXJJ2MTQV39V9P62AE&hash=item26bcb0e6f9:g:-wQAAOSwRZJlJxr2