I am running a more western style campaign and I just had a player join a smithing guild. He is very excited about this as a artificer and wants to explore this aspect. I want to send him and the party on a few quests from his guild to collect rare materials to melt down, rare weapons etc. 

Any ideas for rare materials for him to go fetch? This can be ore, monster drops, or anything else that would make sense!

I have ideas about potentially slaying dragons for their scales but I need more. Any ideas would be helpful! He hopes to actually craft things himself too so I wanna give him opportunities to collect cool things by which to do this. 

Adamantine,  Mithral, Cold iron ,dragonhide, darkwood. Well monsters whole corpse is left so you can really just put in any monster and have them loot the corpse. 

There are cool things that you can steal from Eberron including dragon shards, live wood, dence wood, and crysteel. You could also send them on a mission to secure a mine such as the lost mine of Phandelver.

My campaign so far has darksteel, quicksilver, and songwood as magical resources and monster drops can include hide, bones (fangs, claws, antlers, etc), or essence (which includes stuff like blood or the magical core of an elemental), as well as 'environmental' drops like different types of stone and gemstones. One fairly common thing to do is to associate a gemstone with each element or damage type: fire ruby, cold sapphire, etc.

This wiki has a pretty decent list of interesting materials that you can use for ideas of what the players might be able to seek out (includes the names of the source books for most of the materials):

https://dungeons.fandom.com/wiki/DnDWiki:Special_Materials

Okay this is amazing and so helpful!!!!