My humidor says to charge it w/a distilled H20/propylene glycol mixture. Where do I get these ingredients?

I have never owned a humidor and want to maintain it properly - I'm willing to do whatever the instructions say but "distilled water/propylene glycol?" I never heard of either ingredient and just wanted to get an idea as to what kind of store would sell these ingredients? I figure the place where my humidor was purchase would be a good start but its 45 minutes away (this was a gift). Any help would be appreciated. Thanks!