No designated sledging areas on cairngorm.
The hayfield in Glenmore is a great place to sledge. If the cover is poor then go into coire na ciste car park as George suggested. Stay away from Coire Cas it will be bedlam there!
Sledges available at Bill's shop aka Glenmore shop opposite the Youth Hostel.
Come with TREKnTRAIL for a memorable and enjoyable outdoor experience walking or mountain biking in the wonderful Cairngorms National Park near Aviemore.