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

Steel Washer Zinc Plated 3/8in

https://smile.amazon.com/dp/B000BD5HEA?pd_rd_i=B000BD5HEA&psc=1&pd_rd_w=uAD4v&pd_rd_wg=AghsF&pd_rd_r=VA4NG19DVAEG64GY7K71&ref_=pd_luc_rh_bxgy_01_01_t_ttl_lh

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

https://smile.amazon.com/gp/product/B00CAO5BQW/ref=ox_sc_act_title_3?smid=ATVPDKIKX0DER&psc=1 $5.63