I don't have a clue what process your testing for. Generally, test plates are 3/8" X 3" X 8" for SMAW welding which limits your qualification to 2 times the thickness.
If you just need A36 plate sheared for general fabrication, any steel distributor will do that for ya.
Test Coupons are available at; Dayton , Ohio
Fischer Engineering Company "about any type" call for sizes.
937-754-1750
or
Triangle Engineering, Inc. - West Hanover, MA
617-878-1500 471-1762