you can buy dry crystals from watergelcrystals.com (I think that's the page).
I use them to maintain my instects, but I avoid using them with small crickets destined for small chameleons - I lost a month old veiled to them. It ingested a partially dehydrated crystal - stuck to a cricket, or trasferred to their cage while I was feeding them - and died. I pulled it out of the dead lizard's stomach, swollen too big to pass.
I've had adults pass them with ease - but I see that it doesn't happen anymore, obviously!
Fresh fruits are best, but they go bad quickly, and I'm too busy to keep up with enough fruit for my roaches. Plus, when I go away, I need something that'll keep for a few days - agar goes bad and harbors bacteria. Plus it's expensive and a pain to make in comparison.
I think I got 5 pounds for less than $10.00. That's enough to last my hobby!
I find it to be excellent for maintaining and breeding the insects. When I go to feed them off, I usually give them greens and veggies before hand.