If you have T-Mobile service, they actually have a specific service and device which essentially acts like a cell phone tower, and connects to a broadband Internet service to send the data back to the cellular network. It goes into places like T-Mobile stores in malls where reception is poor, other businesses, places out in the middle of nowhere. I'm not sure exactly how you'd go about getting signed up for the service though.
Other providers might have something similar. I think Cingular does.
Disclaimer: I work for a company doing business with T-Mobile on this specific service. However I'm providing this to be helpful, not to get more customers, because I don't really care if they get more people using it, all it does is add more sites that I have to deal with the support hassles.
Oh yeah, you could also build a wall outside your building on the opposite side of any towers that might provide a weak signal, and coat it in aluminum foil.
🙂
Found
a page about the service, sort of.