Not familiar with HKS coilover assemblies, but here's a solution:
Go to your nearest hardware store and see if they have a caliper that can measure the thickness and diameters accurately
Buy whats called a "flanged sleeve bearing" in the appropriate size. This store sells all sorts of ****:
https://www.fastenal.com/products/fl...0Bearings%22|~
Should give you something close to what you need
if the size isn't exactly right some metal shop can probably grind it down a bit for you as needed
This is if you can't get thru to HKS