improving wi-fi connection/reception no router or modem being used
I'd try one of these:

http://www.radiolabs.com/products/antennas/2.4gig/2.4-aluminum-parabolic.php

If you are not moving around, and can lock on to one of the towers visually, you could point this antenna at it and try to maximize the weak signal. Kind of like a shotgun microphone or a telephoto lens.