Many decisions of the Comptroller General of the United States through 1994 can be found in the Decisions of the Comptroller General of the United States (Comp. Gen.), which are available in PDF from HeinOnline.
For those decisions that do not appear in the Decisions of the Comptroller General of the United States, use the Comptroller General's Procurement Decisions.
These decisions can also be found on Westlaw, Lexis, or VitalLaw.