A cast-off aquarium works well, too. Maybe a recycle arrangement could be coordinated with retailers of new aquariums? I can understand why they wouldn't want to store and re-sell the old ones but, maybe they could suggest donation to Urban Roots, who could then sell them at a very low price? Just a thought.
I too, had a terrarium as a kid (mostly African Violets). I think this is a great hobby for young people… it fosters caring, hope, patience, appreciation of nature — all in one's own micro-world. Good luck with the program!