kegworks.com has a 10g CO2 tank for $79, although in both cases you have to look at the shipping costs as well. They have a table at the link above with tank sizes. The 10g is 6.25"(dia) x 22.25"(tall w/valve).
Most posts I've seen on this recommend getting the tank locally. They must (by law) be shipped empty, so you're going to have to find a supplier anyway to fill it and neither of these prices seem that low when you factor in the shipping.
-carpguy