Bolt spacers

...a local steel company...…..
...or call some machine shops in the area. They always end up with racks of unused material.

1 & 2 ft on goldbay… …….
Steel Tubing /Spacer/Sleeve 5/8" OD X 3/8" ID X 12" Long 1 Pc DOM CRS | eBay

Steel Tubing /Spacer/Sleeve 5/8" OD X 3/8" ID X 24" Long 1 Pc DOM CRS | eBay