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Where to find parts can depend a lot on what specific parts you're looking for. A significant number of mechanical parts are the same for Toyota Matrixes of the same year, so often finding a cross reference to a toyota part can yield a source.
If you're looking for body parts, then maybe the best hope is to try to find them at a junkyard. I often use https://www.car-part.com/
to search junkyards near me for parts, seems like most yards are plugged in to that database.