I obviously don't own Portal 2 yet but am considering buying since it's on sale on Steam at the moment. My question is - I've heard that the SDK has been released some weeks ago so one would expect some custom maps already available. However I cannot find one reliable website with mods and maps. Where can you get them from? Are they available from within the game instead?
On of the best sites for maps and mods is ThinkingWithPortals.com. Consider that at the moment there are few mods because the SDK has been just released and modders need time to create something different, but there are available already many maps.
They announced a couple of weeks ago winners of Summer Mapping Initiative, a contest for map makers that had a lot of entries:
Over the course of the past two and some change weeks, our 13 judges have spent over a collective 140 hours playing 240 of some of the best custom maps this community has seen. As I have said previously, the turn out for this contest has been amazing and I want to thank each and every one of our participants.
In addition you can find also something at Mod DB.