Text Messaging Vendors

For full servive text message vendors, try (in alpha order):

Distributive Networks
Mobile Accord
Mobile Commons
Single Point

(if you have the money, hire a vendor).

Otherwise, you can use lower-tier SMS providers such as:

Clickatell

If you're looking to bootstrap, check out this article I wrote that compares low-budget SMS software or so called Desktop SMS Software. FrontLineSMS is my favorite.