This question comes up so often that there should be a sticky post for it!
Once upon a time, there was a preset-sharing section of the IK Multimedia web page, but it disappeared ages ago and shows no obvious signs of returning anytime soon. There is no official word on why.
At the moment, there is a fan-run Facebook group for preset sharing. It's pretty active, but obviously Facebook is not really a good interface for this sort of thing and not everyone uses Facebook. Still, if you do use Facebook, you should be able to search for the group and find it pretty easily. (It has some fairly obvious name, like "Amplitube Preset Exchange" or something like that!)
There used to be some online forums for general amp sim preset charing (for Amplitube and other software), but I don't know if they're still active.