Dimwitt is correct, most welding supply houses have the lil' bottle of marking paint. You may also get it at White Cap Supply...they are nation wide...............
My sone the welder says you can get those pens at most welding supply shops like the national chain Airgas or maybe a locally run business selling primarily welding supplies.
Best bet would be home depot or some where like that. They are called wax crayons. personally I have spent many years behind a junk yards fence and we never used paint. With paint it is harder for the customer to remove. With wax crayon's it is fairly simple to remove.
