I'd strongly recommend cable or DSL as some AR15er's have mentioned, if it's an option for you.
PC Card modems for laptops have long been able to connect to cell phones, usually with a special cable provided by the modem manufacturer. To use one of these on a desktop PC, you may need to install a PCI card that will accept laptop PC cards. Check [url]http://www.techworks.com/products/airstation-wli-pci.html[/url] for the "Bus Adapter" at the bottom of the page.
As for the modem, check some of the bigger mail order places, such as [url]www.cdw.com[/url] or [url]www.warehouse.com[/url]. Local computer shows are also a good place to find lightly used PC card devices for cheap, but usually without the cable.