I need a little bit of photo help.?

Im in my jr. year of high school and i'll be doing the schools photos for sports that go into the year book. Im looking at getting a D-SLR camera (Canon rebal XT) and a digital lens. Im going to be able to sit on the side lines for all the football games, and ill have pretty much side line rights for the basket ball games, anyway, i need a digital telephoto lens. anyone have any idea on sites that sell them reasonably cheap ( 300 ta around 600) ?