http://www.data-alliance.net/servlet/the-WiFi-CARDS-fdsh-clients-cln-for-Laptop/Categories
I've bought a few wifi cards from these guys. They mostly sell stuff that is for enthusiasts and security folks, and the prices
aren't jacked..
The
Alpha 500mw USB works well, I've used a few of them for increasing range. They are also the darling of the hacking community, as they are fully supported under Linux (and OS X, Windows) and will run aircrack and other security tools.
But just having the range and means doesn't make it legal to crack WEP/WPA networks in your neighborhood! It's one thing to find yourself accidentally connected to an open hotspot, it's another to deliberately crack their security. If you're into security testing, just stick to your own router, it can be highly educational!
Also, all that power does not translate into increased
receiving ability. As in, 500mw will help you transmit a strong signal to the router keeping your uploads at peak speed, but if the router is broadcasting a weak signal back, your downloads may be quite a lot slower and inconsistent.
This is why a good antenna is also important. I use a 9dBi antenna that is still short enough to be carried in a bag. A directional antenna like the
Wi-fire product, might even be better, but you'd have to dig into some reviews to compare performance.
Good luck!