Hi Dave,
Not a film guy, but I spent a lot of time, back in my college days, working in theater as a techie - building sets, running lights, etc.
Gaffer's tape is available from theatrical supply houses and I would assume you could get it from places like B&H, since they have a pro video division.
A friend of mine is an electrician for General Motors and they use it all the time on the job, they call it green tape, it's sort of Olive Drab, but it's the same thing as Gaff tape. It doesn't leave that sticky mess that duct tape does.