I think the word you may be looking for is "Hydrogels." They are water absorbing polymers. You may be able to find them where you can purchase gardening supplies or plants. Some florists may have them. You may want to try Amazon, ebay, or some other online source that has sellers outside the US. Or, perhaps contact the manufacturer. You want to be sure they don't have fertilizer though (as I've been told that some Miracle Gro crystals have them).
I've been looking into breeding dubia roaches, and when I was researching the water crystals. I was concerned about them after reading research articles on the consumption of hydrogels. The more I researched, the more I became concerned, so for us, we won't be using them (unless we find we must). But it's up to each person to decide if they feel they are safe enough or not.
For our crickets, we just always provide fresh fruit and veggies and they get their water that way. From what I've read, if they are provided with fresh fruit and vegetable items (such as potatoes, oranges, etc.). I do sprinkly water into their contain as well.
