GE's Silicone II caulk is readily available, and suitable for electronics use (no acids released during the curing process that would cause corrosion; it uses moisture from the air instead to catalyze the curing process). Easily found in most hardware stores, Wall*Mart, and of course the big DIY box stores such as Home Despot or Blowes.

Not expensive either.

Although it seems less cost effective, the smaller tubes can actually reduce waste from the standard sized tubes for a caulking gun when it hardens in the nozzle/tip (i.e. 2.8oz tube, which runs ~$4; larger 10.1oz runs ~$6.50).