you will need three supplements. Get calcium without D3 and use this at every feeding to dust your feeders with. Also get Calcium with D3 and use that a couple of times a month. Lastly, you will need a multivitamin that you will also dust the feeder with twice a month. Repcal and Sticky Tongue Farms are the brand that I use along with many other forum members.
Exposure to UVB from either direct sunlight or a proper UVB light allows the chameleon to produce D3 so that it can use the calcium in its system to make/keep the bones strong and be used in other systems in the chameleon as well. The UVB should not pass through glass or plastic no matter whether its from the sun or the UVB light. The most often recommended UVB light is the long linear fluorescent Repti-sun 5.0 tube light. Some of the compacts, spirals and tube lights have caused health issues, but so far there have been no bad reports against this one.
Since many of the feeder insects have a poor ratio of calcium to phosphorus in them, its important to dust the insects before you feed them to the chameleon with a phos.-free calcium powder to help make up for it.
If you dust twice a month with a phos.-free calcium/D3 powder it will ensure that your chameleon gets some D3 without overdoing it. It leaves the chameleon to produce the rest of what it needs through its exposure to the UVB light. (Some UVB lights have been known to cause health issues, so the most often recommended one is the long linear fluorescent Repti-sun 5.0 tube light.) D3 from supplements can build up in the system but D3 produced from exposure to UVB shouldn't as long as the chameleon can move in and out of it.
Dusting twice a month with a vitamin powder that contains a beta carotene (prOformed) source of vitamin A will ensure that the chameleon gets some vitamins without the danger of overdosing the vitamin A. PrEformed sources of vitamin A can build up in the system and may prevent the D3 from doing its job and push the chameleon towards MBD. However, there is controversy as to whether all/any chameleons can convert the beta carotene and so some people give some prEformed vitamin A once in a while.
Gutloading/feeding the insects well helps to provide what the chameleon needs....so its important too. I gutload crickets, roaches, locusts, superworms, etc. with an assortment of greens (dandelions, kale, collards, endive, escarole, mustard greens, etc.) and veggies (carrots, squash, sweet potato, sweet red pepper, zucchini, etc.)
Calcium, phos., D3 and vitamin A are important players in bone health and other systems in the chameleon (muscles, etc.) and they need to be in balance. When trying to balance them, you need to look at the supplements, what you feed the insects and what you feed the chameleon.