Dandelion is incredibly easy to grow - its a weed that grow year round here in BC and I suspect most of Northern America. And where it doesnt grow like weeds yes many grocery stores will sell it.
If you are going to eat it yourself, get fresh Spring leaves - They are a bit bitter in the Fall and Winter.
Alfalfa is another choice for B Vitamins - its an easily digestible source of protein, B-complex vitamins, calcium, phosphorus, magnesium, iron and potassium.
Yams / sweet potatoes are possibly better options than every day potatoes.
I actually suggest switching up your gutload, changing it periodically. I feel this helps one ensure a well-rounded nutritional content. And it probably makes the insects taste a little different, so the chameleon doesnt get bored.
Some of the things I use as gutload include: dandelion, kale, endive, mustard greens, carrots and carrot tops, squash, zucchini, sweet potatoe, celery leaves, alfalfa, spirulina, sunflower seeds, coconut, orange, ...