Does anyone know whether there's any way to get a hold of the content from previous D&D Encounters seasons? I'd like to be able to try some of it out.
Ask around- both at the shop and on the mailing lists used to organize Encounters (if they do that in your area). The thing is, usually the DMs for the encounters season are allowed to keep the kits, and they may have an extra at the shop they'll let you look at or have, or one of the other DMs may be willing to give it away. It doesn't hurt to ask, anyhow.
In the past I've given away freebie material like this myself- the set of ship tiles I got for DMing RPGA stuff, as well as the 4e Village of Hommlet all went to some random person asking if he or she could borrow it. So I bet other people wouldn't mind.