There are plenty of businesses around San Andreas that offer CJ's services. These range from fast food places, to clothing stores and even to mod garages. All of them are important to CJ in one form or another, so it's imperative that you learn about them. Note that all places of business in Las Venturas are 20% more expensive than in other areas.
Firing or pointing a gun at anyone inside any of the stores will negate any chance of you shopping/eating there while you remain inside. Also, do not aim at the cashiers at any place in order to maintain business. If the red marker disappears, simply exit and re-enter the place to become a customer again.