Do you need Xbox live to get them? I don't have it. :embarrassed:
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Actually, it's completely the other way around. You can't map or get public maps on Xbox, only PC.
You just have to download the Source SDK in your "Tools" tab. The Valve Hammer editor is rather difficult to use though, and you'll need to read a bunch of tutorials.
I suggest you check the tutorials for CS Source on Egir. The CS tutorials should work too, since the editor for the two are basically the same.
Actually, halfwit-2.com has some nice portal tutorials.
http://halfwit-2.com/?page=tutorials&category=17