Vitamin and nutrition stores, such as GNC, usually have en entire shelf or two devoted to Vitamin A. You can also get it on-line.
The primary Vitamin A compound out there is retinal palmitate. It is a fat soluble vitamin, and as such, is usually dissolved in an oil, in gel caps. When you look at a Vitamin A product on the shelf, you should note the Vitamin A source, wanting to see retinal palmitate, and the concentration per gel-cap. Different products will range from 2000-15000 iu's per gel cap. If beta-carotene is the source, skip it. It will not work on your chameleon.
We recommend that you dose at approximately 200 iu's per 100g of animal every couple of weeks. It is a very inexact science. Should your animal already be showing signs of Vitamin A deficiency, you would dose it daily for 5-7 days, expecting to see improvement around day 3-4,, except in severe cases, where the wait may be longer.
Chameleons do not like oil in their mouth. The above dose rates equate to a smear with a moistened Q-tip, either directly into the mouth, or on a cricket that is then eaten, which is not enough to cause a negative reaction. However, administering a drop or two of any oil into the mouth can sometimes induce vomiting, and other undesireable results.
That said, for an animal showing Vitamin A deficiency, I believe it was Dr. Ferguson who would often dose up to 4000 iu's at once for an adult male. I am not sure how he did it while keeping the oil volume low, but he had success with it.
It is also possible to cut the oil with with bipolar solvents such as glycerine or propylene glycol, to make it water soluble. Liquid vitamins, such as children's Poly-Vi-Sol, use glycerine.
Good luck.