What kind of systems are you talking about? Windows? Linux? Do you have an existing network?
How fast a connection do you want? You can channel-bond with ISDN to get a somewhat faster connection, and given that the technology is losing favor,the likelihood is that you won't be sharing the line with enough people to interfere with your connection.
Routes you might want to investigate would be (a) some form of DSL; (b) a broadband cable modem at each location, with a separate card in each system to provide a private network; (c) a wireless WAN.
You can either purchase a cable modem, or lease one from your local provider. If you have any familiarity with Linux, a web search for "wireless WAN" will give you pointers to How-to's for the process.
-- LJM