In the past my friends have usually just looked around the village...there quite a few locations where you can just walk in, and they had no problems finding B2s or B3s. The prices were a lot better than what demos went for in Tahoe.
Tons of shops, Summit, Snow Covers, Affinity, and even Can-ski (and a few others), but Rossignol is very well represented - you should have no problem. I've always had good dealings with Snow Covers and Comor.
Whistler has on the hill demos (free) as well. You can check with Canski in the village as to their demo schedule. I believe that they offer then demos pretty much every weekend. There are also lots of sales on gear right now, so it will be good time to buy.
Also a lot of the shops will deduct the cost of the demo from the price of the ski if you buy.