1. any item, found or obtained for free.
2. materials used to create art work such as assemblage, mixed media or sculpture that wasn't bought new, but obtained in other ways, such as second-hand, dumpster diving, chance findings or donations.
Building good terrain relies heavily on obtainium. In most cases it is found as raw materials like foamcore, craft sticks, etc. Other times it comes as finished products by companine like GW and Woodland Scenics. Sometime obtainium comes in even stranger forms.
The parts are out there, you just need to obtain them.