I do not know about selling but this places does rental, with about twenty catalogs of all kinds of outfits (including Tibetan) to choose from.
Hong Xiufang（costume rent shop）
Address：No.28，Beimen Street（Shop No.30 in Cuiming Yuan)
If you really want to buy chupas and stuff, then PM me and I can recommend a few authentic places in Lijiang and Shangrila.
@Ifoundthetuna, sorry for the inconvenience caused. As already discussed by phone we've once more been facing internet problems last week and we lost several emails without noticing it. We very much appreciate feedbacks like this so we can follow up the issues accordingly and re-establish our high service level.