An original? Not sure I understand. I would say I would never buy a baby anything expensive or over the top, one of a kind of anything unless it is just a keep sake, with puke, slobber, messes it doesn't make sense. I wouldn't pay more then $8 for a tshirt at that age nor $8 for a onsie!, I think you can get a three pack of onsies for $8 at Target.
If it is a tshirt from some place you vacation, then I would get a larger size so they can wear it later and when they are less prone to staining their clothes. Just my two cents.