Strings Direct sell Alaskas too, I think. Don't forget to get a spare (or two) for when one drops off and someone stands on it...
Be warned, the plastic ones aren't that well finished - a fair bit of flash left, which is uncomfortable, plus the edges are quite sharp. Use a file (nail file works) to smooth them off, and they're a lot more comfy.
Johnny's right about them needing a reasonable length of nail to start with. They do give you a bit more volume and a different (plectrummy) tone though. If you're in a noisy folk club without an amp, it lets you get away with fingerstyle instead of being limited to strumming for the volume.