I think autozone will let you buy any paint you want, You should be able to buy undercoating for sure.
Walmart thinks they are helping the community by not allowing kids to buy anything aerosol, glue, 3 in one oil and anything that smells.
Most everywhere else allows you to buy paint I think. Walmart goes the 'extra mile'
I know that at home depot that they wont let me buy the 99cent paint, But I can buy Krylon Fusion spray paint.