I frequently see floor mirrors at Ross and TJMaxx. I'd check there first. Also Home Goods and Marshall's.
If you find one that isn't white, buy it anyway! Mirror frames are SUPER quick and easy to spray paint. I prefer more of an heirloom/antique white, but if the PBTeen one is bright white, just buy regular white spray paint.
Edited to add: Oh, after looking at PBTeen I see you mean the style with the legs. I assumed you meant the giant mirrors that you prop up against the wall at a slight angle. My mistake. Usually furniture stores like Furniture Row, Mor Furniture For Less, etc offer mirrors like that for around $30 this time of year.