The issuing agency is always either the House or the Senate.
U.S. [Senate/House]. [nth] Congress, [nth] Session. [Bill number], Bill title, may be shortened. Edition statement. Washington: Government Printing Office, year.
Always use the bill number in the title. The edition statement, if applicable will either be "Act" (if "act" is in the bill's title), or "Star Print" (if there's a little black star in the lower left of the title page).
U.S. House. 75th Congress, 1st Session. H.R. 6786, A Bill to Protect Purchasers of Hats. Washington, Government Printing Office, 1937.