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.
Once you've collected your mobile numbers, verified that you have followed all of your legal responsibilities you can start your SMS marketing campaign. I'd suggest using a 3rd party service provider to ensure that you are able to track the return on investment for your campaign, embed an unsubscribe message, and easily manage your SMS marketing database including unsubscribes.