Steel Washer 3/8"
It is actually better to use 5/16" washers for 3/8" bolts and threaded rod.
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)