Plastic Fasteners
 

Would these work for a 2005 Mazda rx8? Ford & Mazda Push-Type Trim Retainers B100-68-675-69

Probably. For what exactly? Whenever I need those I just bother the guy at my dealers parts counter and he gets me a bunch of the correct type for cheap.

Lots of autoparts stores have "help" sections with lots of different generic items like that. You might be able to match them up.

There are at least three different (that I can think of) fasteners of that type on the car. If you're looking for replacements for the undertray, then the part number you are looking for is

BC1D-56-145

Mazda Mud Guard Retainer 20mm Hd Dia 9mm St Lgth

You don't really have the use the original fasteners if its a hassle to find, you could use the ones almost the same size as the original...if it fits, then its all good.