You can use the online EPC (Electronic Parts Catalog) to find the part number for everything:
Spare parts online electronic catalogs for Toyota, Lexus
Select the Toyota | USA selection in the upper left. Then "MR2". Then "AW11L". Then select any of the "complecations". Any will do since the body stuff you're interested in is common to any 87+ MK1. Navigating to find the right section can be tricky at first.
Most of the nuts & bolts are still available since they are "generic" parts used on many models.
There are wedge shaped plastic screw inserts that are used for the side skirts (4 per side). I think the part number is 90189-06064. They're critical to mounting the side skirts properly, so don't use generic ones there.
The black plastic/rubber molding strips that go between the side skirts & car body should be removed before painting the skirts, then re-installed later. Having them sprayed body color just makes the car look like it got a quicky repaint w/o masking them. Same goes for the molding strips on the front spoiler