Would there be some weapons that would be hard to come by in the underdark? Just as there are weapons in underdark that you might not see on the surface?

I hope the DMs, perhaps in the interest of fun over realism, make it so that you can acquire any weapon from the surface, and the even more unusual and unconventional weapons that might be found in the Underdark.

It might be harsh if someone conjures up a killer concept with a weapon being pivotal to the character and there's none to be found.

Some weapons are more rare than others (i.e. you might need to find special merchants), but every kind of weapon will be in the game.