Insects do have vitamin A...in their eyes....so they must get it from the food they eat that already has prEformed vitamin A in it or in the case of strict vegetarian insects, they must convert it from the beta carotene in the veggies, greens, flowers, etc. that they eat.
However...there is controversy as to whether any/all chameleons can convert beta carotene...so gutloading the insect will provide the insect with beta carotene...but its still not known if the chameleon can convert the beta carotene in the insect's gut to vitamin A.
For preformed vitamin A, which I personally do not think is necessary in great quantity, you can gutload your crickets with a hard boiled egg, animal liver or dog food. The highest concentrations of preformed vitamin A are found in moose and walrus livers. Note that I am merely answering the question asked, not recommending you use these as gutload items. It would do no harm to use one of these in a small way on rare occassion, perhaps annually, but it is possible to do harm if animal meat is provided to chameleons (directly or indirectly) on a regular basis.
For beta carotene, good sources include: sweet red peppers, carrots, paprika, grape vine leaves, dandelion leaves and butternut squash. And Spirulina algae
Vitamin supplement powders also contain either preformed vitamin a or carotenes or both.
Be mindful of Kinyonga's comment as well - there IS more to it