Graffiti Supplies

I never knew that they made spray paint specifically for graffiti. In fact I didn't know there was a need for such a product. A company in Barcelona Spain called MTN (Montana) makes specific markers and spray paint for graffiti and there is a store called Art Primo where you can buy spray caps designed to give you specific lines used in graffiti. Since graffiti is illegal in most places I wonder how these products are sold openly?  And since graffiti is a street activity that uses everyday items, I wonder if there is really a market for this.

Does anyone have more information?