I have a pretty good sized mealworm colony going and I use carrots and potatoes. Replace every two or three days (depending on when it starts to get moldy) and you should be good to go!
I use carrot chunks, but I have also used a wet papertowel. I have those select-a-size that are really half sheets, wet the top half and fold it over, so the wet part doesn't touch the bedding, and therefore doesn't clump.
I also have some water gel and I have been testing it, but idk if it's a good source yet (I would suggest putting it on a small piece of paper towel)
I've always used carrot chunks for both meal and superworm colonies. The hundreds of beetles totally devour them, so there's no mold or waste at all. Beetles have a decent life span too. My produce market has giant carrots that weight over half a pound each @ .75#, so 1 carrot lasts several weeks to a month. The worm bins get other veggies and fruit.