Look for items that have the required shape or negative shape for use as a mould.
Easter Egg packaging might serve as the shell of a pod or form part of a chair.
Consider the ease of construction, transportation and display.
Models are often broken during transport to shows, don't mix heavy parts with light constructions.
Consider the rigidity and scale of the finished model parts.
A larger model might need stiffening by the use of a wood or metal form, a visible wall will need the correct scale thickness.