The "other" method is simply a damp sponge that is put inside either a plastic baggie or a plastic soap dish - both with holes cut or drilled to allow the moisture to escape.
Serach on plastic soap disk and you will likely find many posts, even including a "how to" guide, although it's pretty straight forward.
The expense of replacing several Humidipacks in several guitar cases a couple of times a year just seems a little wasteful to me, not to mention environmental concerns (no, I am not a tree hugger
). What the heck is in those things anyway?
If your going this way, it helps to get those "mildew resistant" type sponges that do seem to be available in any large grocery store. The soft plastic soap dishes are easier to drill holes in than the hard plastic ones. I have been using the current small squares of sponge for well over a year, and they don't smell yet!
I have also "tested" this with a known good hygrometer in my cases, and for whatever reason the humidity levels land squarely in the safe zone.