Asbestos and Lead are regulated substances in alost every state. Sampling for them requires special training and licensure in most cases. I would seriously consult with your states environmental authority before just going out there and offering these services. There can be some significant fines associated. Radon is also regulated in many states as well.
If you don’t have training in these areas and are going into environmental testing without any kind of knowledge, you can set yourself up for some significant liability. I would suggest looking into taking a few courses on each subject before offering them to the general public. Selling environmental services as part of a home inspection takes practice and a solid understanding of each service. Giving out wrong information or not revealing certain facts can make you look unprofessional and effect the health of a home owner.
Take as many course as you can on a subject so that you get different opinions and perspectives. I would suggest paying for a membership with ESA and taking their courses on mold, radon, asbestos & lead awareness.