This question exists for many. First, NEVER EVER purchase a list of mobile numbers. (SMS/MMS) marketing is permission based, and ultimately you want a list of customers numbers who are genuinely interested in your product or service and are happy to receive your text message and are likely to buy your goods and service. Remember, you can turn customers off and possibly upset a lot of your potential customers when sending text messages on their mobile devices without their consent.
You will have a greater chance of gaining access to their mobile number by following these guidelines. You can collect their numbers using a sign-up sheet at your register, asking them when talking on the phone, allowing them to submit a form on your website, or by giving them a number that they can text to subscribe to your SMS messages. Whatever you do, keep a copy of their permission to market to them via SMS, if you run into problems you will be glad you did.