Two ways we do it...
One, in March, we go to ski shops and by clearance. We can usually get about 70% off, but the selection is often poor, particularly for kids.
Second, a lot of people we know (we haven't yet) go to ski swaps - usually held in October. Basically, a huge garage sale for people buying and selling old equipment.
I wouldn't buy anything over the internet. Shipping costs will eat into savings unless you find it local (Craigslist) and you'll want to look at it. Also remember that you'll need to have bindings fitted, so there will be that expense in anything you buy used (or frankly, new, although some places through in the labor when you buy new).