Apple now sells the wheels and feet for the acclaimed Mac Pro 2019 as separate kits. But like the Mac Pro itself, both kits are quite (if not fantastically) expensive.
If your Mac Pro sits on standard feet and you want to increase its portability, the $ 699 Mac Pro Wheels Kit gives you four stainless steel wheels. It should be noted that the set of wheels will be cheaper if purchased as part of the configuration of the finished Mac Pro - so they will cost "only" $ 400.
The $ 299 Mac Pro Feet Kit, on the other hand, provides four feet - also made of stainless steel. They can be installed in place of wheels to keep your Mac Pro in place instead of rolling.
Apple says both kits come with 6mm to 4mm hex nuts, but "extra tools are required" to install them. Both sets are already available to order.