Are you talking about the same type of retainer [screws] that hold up the Mach 1 chin spoiler? Those black ones with the flat head that you just push into the hole? You can buy those from Jegs and such, or prolly at Auto Zone even.
If youre talking about something other than plastic, I would suggest FIRST simple metal studs, only cuz Ive seen them used before, though you may wanna get some that are pretty thick. Worked on my buddys car when he added a Saleen kit to his new edge.
My easy way out of things? I just use simple screws and nuts to hold parts of the body onto the frame. Of course, I dont buy screws with a HUGE head, so that you can notice them right off the bat, real tacky.