This is the first time I've seen them:
https://www.ebay.com/itm/10-ENNIS-C40-Stimsonite-Reflective-Road-or-Driveway-Markers-Red-White/362144942997?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D49451%26meid%3D3cd8916fd882458288b9f1227930298f%26pid%3D100678%26rk%3D1%26rkt%3D15%26sd%3D362144942997&_trksid=p2481888.c100678.m3607&_trkparms=pageci%253Aa35b3f45-de9c-11e7-b829-74dbd1805b50%257Cparentrq%253A46b8b9821600a99bc700ecd9fff24b93%257Ciid%253A1 (https://www.ebay.com/itm/10-ENNIS-C40-Stimsonite-Reflective-Road-or-Driveway-Markers-Red-White/362144942997?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D49451%26meid%3D3cd8916fd882458288b9f1227930298f%26pid%3D100678%26rk%3D1%26rkt%3D15%26sd%3D362144942997&_trksid=p2481888.c100678.m3607&_trkparms=pageci%253Aa35b3f45-de9c-11e7-b829-74dbd1805b50%257Cparentrq%253A46b8b9821600a99bc700ecd9fff24b93%257Ciid%253A1)