Steel Washer 3/8"
Depending on the manufacturer it can be better to use 5/16" washers for 3/8" bolts and threaded rod. The 3/8" rod fits through them fine but with a better tighter fit.
The Hillman Group 830506 Stainless Steel 3/8-Inch Flat Washer, 100-Pack
5/16 in. Zinc Plated Cut Washer (100-Pieces)
http://www.homedepot.com/p/Crown-Bolt-5-16-in-Zinc-Plated-Cut-Washer-100-Pieces-46050/203639702 $9.95 for 100 (10 cents each)
It can be useful to also add a split washer to avoid having to also put a spanner on the far side.
The Hillman Group 300024 Split lock Zinc Washer, 3/8-Inch, 100-Pack