Well, if a player beg a DM to buy or make potion flasks or other items, as easily handled as weapons, then it is up to each DM if allow it or not IMHO. And it may be a new kind of items better handled by a specific house rule. Because nowhere in the rule book they are treated as weapons. Maybe you can apply some extra cost for such modification.
Regarding shuriken, well, I am not truly sure if it is realistic to make star-shaped or disk-like shuriken is easily drawn. Or even truly practical as a weapon. Even here in Japan, it is debated again and again by historians and martial artists if such shurikens are really practical. FYI, most of the famous Shuriken-do styles are using dart or needle-like shurikens. Not disks or stars-shaped ones. Also, as pointed out by many from the beginning, spiked chain does not seem to be realistic, so as most of the double weapons. And Kusari-Gama in DMG is not at all like true Japanese Kusari Gama (Kama part is not for throwing, so it should not be ranged slashing weapon). So, I am regarding star-shaped shurikens in D&D (and many other RPGs) as somewhat "fictional" weapon. I don't think much about "reality" when thinking about Star-shaped shuriken in RPG. It is like not thinking about if wings of dragons can really support their weight.