I just listened to the "Hop Shortage" show on BBR.com, and Chris Colby said Santiam is a good sub(I planted two rhizomes of this last year, but didn't get any hops off it b/c I planted super late, hopefully next year I'll get a bunch). He also said a hop called Vanguard(sp?) is a good sub for Hallertau. He also said both of these should be available.
Yeah being a relative novice and not knowing what the difference is between most of the hops, some sort of thread or Wiki post that lists the closest alternatives for hop carieties would be very helpful, especially as the shortage limits our options.
Fuggles are something like 3.5 to 5.5 AAU and and Tettnang are 3 to 5 AAU so that would most likely be ok in terms of AAU. However, Tettnanger is a bit more spicy than Fuggles in terms of flavor profile (Fuggles are a bit more mild from what I've read but I'm sure that this is going to vary from source to source).
Just my two cents but I'd guess you could get away with it, depending on the style. You may get more variance with some rather than others. Using Tettnang in an ESB might not turn out as good as using the Fuggles in a more traditional German lager that just lacked a bit of the spicy bite that the Tettnang would offer.