Hey Magnus, glad you're going to get into ski-o! You should look into coming to Tahoe this winter for the
US and North American Championships, it's going to be awesome! It's also the first round of World Cups, first time North America has ever held a ski-o WC!
As for skis, I use regular skate skis, and it seems most of the other countries also use regular skate skis. I've heard of some people using shorter skis, but you lose in speed what you gain in maneuverability with short skis.
Around Boston, the best place to buy skis is at
Bikeway Source, in Bedford. Chris Li, their ski guy, is super knowledgeable, and very helpful. If you aren't interested in buying skis from a local shop, I'd recommend getting them through a ski-picking service such as
Boulder Nordic or
Caldwell Sport.
If you're less interested in speed, and more interested in a pair of rock skis you can beat up, you can search all sorts of online dealers and garage sales, but don't expect top speed from those skis - sometimes you get lucky, but more often than not, the skis being sold on sale are the ones that a knowledgeable staffer deemed unfit for sale to full-price-paying customers.
Good luck!