Try finding a plant called Malabar Spinach,
Basella alba / Basella rubra.
It's not a true spinach(which means no oxalates!), it's a vine from South India eaten as a vegetable, the leaves are loaded with vitamin A, B, C, calcium, iron, manganese, and other minerals. It grows like wildfire and will vine out all over the place, so not only do you get a good food source but your animal gets all kinds of new stuff to climb on and explore.
My veiled absolutely loves this, I grew a pot of this up to have about 10 feet worth of vine and he ate it down to bare stems within a month and a half, then started biting parts of the stem off! Made him regular as a clock too.
I'd recommend having more than 1 pot of it growing at a time(makes a pretty houseplant if you have a south or west-facing window, especially if you get the
rubra species with red stems) if your cham loves his veggies as much as mine does. You can propagate it by snipping off a stem and soaking the bottom portion in water until some roots start sprouting out, then put it in nice moist fertilized potting soil. It will re-root and take off from there.
None of the big box stores are going to have it, call around to the specialty nurseries in your area to find some. If you can't manage to locate it that way, go find an Indian grocery store and ask if they have fresh Alug Bati(Ahh-loog Bah-tee) for sale, or know a place that does.
edit: Here's a link to the nutritional value of cooked malabar spinach (data comes from USDA analysis so it's pretty legitimate), raw will of course have higher levels of certain vitamins:
http://nutritiondata.self.com/facts/vegetables-and-vegetable-products/3049/2
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