I disagree with the answer above if you are talking about establishing a direct 802.11b connection with your ISP.
If so, I wouldn't be able to make a recommendation without knowing the distances involved. For the long hauls (up to 20 miles, you'll need parabolic dish antennas with typical gains of 21db. For a shorter haul, a yagi array might be suitable and for even shorter distances, a directional patch antenna.
I'm a big fan of Cisco wireless gear but if you need to go cheaper, D-Linkhas worked pretty well for me and a couple of their products permit the use of external antennas that I assume you will need. You might want to look at their DWL-1750 with an antenna like the ANT24-1801.
regards,
paul