How can I replace missing parapet wall "joint caps"?
On the jobs I have done like this we used scraps and folded them with duct work pliers (wide flat jaws). If they were fastened at all it would have been on the underside with a small #4 or #6 nail on each side to hold them in place, but usually just a bead of calking to seal the seam and nails that hold them in place. Don’t nail through the top or you have an ugly spot again. 

I would check around any place that does siding and take a photo of your material with a white piece of paper in the shot for color balance. They may have some pieces of a similar material and may cut you a couple of strips for $10. I have gotten pieces for next to nothing in the past for being polite and just asking if they have something close. Good luck.