Mostly grasses and weeds. The best store bought veggies would be spring mix. I also buy dandelion greens often now (its a favorite and good for them, but took me a long time to find out that many stores sell it), kale, mustard greens, and other dark leafy greens. Very Very rarely feed fruits, I really don't think that they need to ever eat fruit and its not good to give often. My Sulcata is about 2.5 years old and I think he's only had fruit about 5 times his whole life. The only exception to this would be pumpkin, definitely not a staple but some people do give it every so often because its said to prevent parasites (but not cure them). Also aloe and prickly pear cactus are good to feed every once in a while. Some people also feed mazuri a couple of times a week. Just remember that variety is key. Don't end up just feeding the same one thing every time.
And thanks for the input Yvonne and Tom