Regular dark, leafy greens should be fine. Those have enough fiber for them. You can try cheese grating up some bell peppers and carrots, but you don't need to worry too much about additional fiber beyond what's normally found in leafy greens.
People use fiber to stay regular, so if your dragon has issues in that area, try canned pumpkin. Even half a teaspoonful will cure constipation in no time. If your dragon doesn't have any constipation problems, just the squash, greens (no lettuce!) and the occasional dandelion flower (my Dubs loves them!) will be all the fiber they need.