For most UCI researchers, HeinOnline is the best source for finding a presidential document when you have a citation.
Presidential materials include:
These documents might not signal legislative intent expressly, but they can indicate a law's importance, a view of its application, and political context. Signing statements may be particularly relevant, since a bill requires the President's signature to become law.
Most of these documents are collected in the Daily (formerly Weekly) Compilation of Presidential Documents. These materials are then published annually in the Public Papers of the President, along with certain Federal Register materials.
See Bluebook Rule 14.7. Example:
Presidential Statement on Signing the SAFE Port Act, 42 Weekly Comp. Pres. Doc. 1817 (Oct. 13, 2006).