When something is sold out in Ubiworkshop it usually stays sold out unless the community shows a great interest in it being restocked. Even then, it is not guaranteed that Ubiworkshop will restock the requested item(s). If you really want something, try checking ebay or sites like that. Also you can try to make a thread in the forums to see if anyone has an item that you want that they are willing to sell (I’m not sure if this is against the rules of the forums so make sure that it is not before doing this).
I know how you feel though, there have been several times when an item I wanted was sold out or the size/color of that particular item was sold out. Sometimes I have been lucky and the item was restocked, but most of the time I just resign to fate and wait to buy the next cool thing they come up with before it gets sold out. You can try to politely request a restock, but it is highly unlikely that Ubiworkshop would restock a product just for one person.