I don't think you can buy those pieces online.. The sill plates on the door jambs you can, but I don't think the rears are available.
As for your carpet, install the rear first, and cut the holes for your seat belt bolts. I installed the bolts to hold the carpet in place. Then I trimmed it on the sides just enough to get the sill plates over top. Same with the front piece, but remember to cut holes for your seat bolts too.