Hinged so one side of the lid can flip open when the other side stays shut. That way if it gets knocked over while open, there is less space for them to escape.
You will have to order them from online. Check the sponsors here. I get mine from mulberryfarms.com or from my local reptile shop. Large petshop chains like petco or petsmart wont have them. I had to find a small mom and pops shop.