can I simply copy the source and slightly customize it to my circumstance?
Yes. Many people have the misconception that originality in and of itself triggers copyright protections. In reality, originality is only a requirement for copyright protection, a protection that is intended only for a roughly defined range of purposes.
Section 5 of the Canada Copyright Act protects "every original literary, dramatic, musical and artistic work [meeting at least one of a list of conditions]", this phrase referring to "every original production in the literary, scientific or artistic domain" (see section 2). The purpose of affidavits is not literary, scientific, dramatic, or artistic.
Similarly, under U.S. law the primary purpose of copyright law is "To promote the Progress of Science and useful Arts, [...] to stimulate artistic creativity for the general public good", Fogerty v. Fantasy, Inc., 510 U.S. 517, 526 (1994) (citations omitted). The production of affidavits are not for the sake of artistic creativity, and usually not for the public good.
More generally, homogeneity in the drafting of legal documents (including affidavits) tends to streamline the administration of "justice". It helps the court to focus on the central issue instead of having to delve in what subtleties could arise from the particularities of how each variation is worded.
