If you want to buy a single class-C IP you should just look for an ISP in your area. Make sure that they handle business clients, and be sure to tell them that you need a static IP (one that never changes).
It's usually a good idea to pick an ISP in your area because its much easier to deal with them vs. a company on the 'net. It's also a good idea from a support standpoint. If you have them do the installation, than they will support it. And if they're local it won't cost you a fortune.
Hope that helps!
Mike Johnson
IS Support Specialist