Currently, the Rarity 1 monster materials are sorted in such a way that, when you sort your Item Box by “Type”,
The first Rarity 1 material of small monsters and large monsters are for weapon, and
The second Rarity 1 material of small monsters and large monsters are for armour.
As such, when disposing of materials, you can consider sorting by “Type” before selecting which material to dispose.
If you’re not already doing so, you can mark the desired equipment as a Favourite, which would also mark the materials needed for upgrading that equipment with the green marker on the materials. This should help you in scenarios where you only want to forge and upgrade the armour pieces of a particular large monster but not their weapon, which should help you in identifying which material is needed by looking at the materials with the green marker.