I bought mine at a Petsmart IIRC. But I've seen it at a Walmart before (but not all Walmarts carry it).That stuff is actually called Fresh Step® Crystals and it is hard to find. Says Crystals right on the front of the bag.
I have one too, but the "capacity" sucks... and for that price you can get much more silica gel by itself and just use your oven to recharge it. No need for a built-in heating element and a large up-charge...I use the Remington rechargeable unit. I think I have the 500 model. When the indicator turns from blue to pink, you take it out of your safe and plug it in to recharge it, then put it back in the safe. Works pretty well.
Just drill a hole in it. It needs to be bolted down anyway so it's going to have holes. Closing the door on a live wire sounds like a really bad idea Deadeye. All of you guys talking about safes floating, you really don't have them bolted in? Making it pretty easy for thieves aren't you? The best price on rechargeable dessicant packs are any of the end of the world survivalist dealers on the web. I bought a large bag of them years ago and they're still working. Just pop them in a warm oven overnight and they come back ready to work.