trophymustang.com has been the best for me....cheapest prices....cheapest shipping...no tax for me (dont live in IL or whatever).
try mustangsplus.com too....not the cheapest, but chances are you wont go wrong.....
sacramentomustang.com
mustangpartshouse.com
mustangdepot.com
i could go on...there is sooooooo many classic mustang places online....a quarte panel is about the most abundant of the replacement sheetmetal....and they definitely make that piece between the glass and trunk...that has no real name....."rear cowl"???? i dono....
anyways, u shouldnt have any problem......