This sounds like an interesting (lot of work) project.
For things like turn signals, headlamps, grills, bumpers, front fenders and hood (normal bolt-on stuff) you can find them around in a lot of shops. Search the I-net for "Sidekick Fender" for your best luck... then follow their links to Samauri parts. You can also find them on Ebay (sometimes) and usually for cheaper at your local parts store.
For a 'Kick, you can pick up front fenders locally for as cheap as $34... a bit more for Sammy parts. The online stores sell 2 lines. The cheap ones (as above) fit some models perfectly, but need some work for other models, at least on the 'Kick. (The 'Kick was made in various countries.) The cheap parts are Chinese, from the markings. The more expensive units (about 3-4 times the cost) are more like dealer items.
For the floor and side panels, you'll need to weld & they haven't attracted aftermarket, it seems. For parts that show, you might be able to repair the hole, then cover with armor. Some places sell the armored pieces, or my friend had some made at a local bumper manufacturer.
For repairing the holes, use sheet metal and weld it into the opening. For places that don't show, you can even pop-rivet it in place. A friend bought some diamond plate, cut it himself, then pop-riveted it on his floors. The VW dudes have found POR-15 works great for preventing future rust.
Good luck.