The question is... how is the teen going to use the sleeping bag? Is the teen going camping in a tent? If so, you may need a true camping sleeping bag for warmth in colder temps or lighter weight for higher temps. If you want a true sleeping bag that is. Then you want LL Bean, REI and true camping stores.
Now if the teen is using it for sleep overs or youth camps in a cabin that is temp controlled, you can look at websites such as Pottery Barn Teen. It may be a little young but The Company Store and Lillian Vernon has kids size sleeping bags that are cute.