My Mother In Law bought all the families last Christmas some L.L. Bean - XXL sized commercial grade snow tubes...Spendy, yes...about $150 each, but it is the most fun, best built, quality, safe, and fun sled we have ever had (and we have gone through at least 75 sleds (my kids usually have about 10-15 different types). The whole family can use it and a few kids fit in it at once. It is fantastic!
I did find a smaller similiar (much cheaper) version:
http://lookmommy.blogspot.com/2008/11/lands-end-snow-tube...
PS: Sleds vary greatly by hill and snow conditions... The sled the kids fight over one day may be the one nobody wants to ride the next. If the snow is deep you want one kind, packed or wet another, soft, icy...you wouldn't think it would be that complicated :). The tube I described above works great in all conditions.