It is pretty rare to find a Prime standing rib roast.
Those folks that sell them,often dry age and air ship.
Most major grocery chains,and Sam's/Costco sell choice standing rib around the holiday .
The stores are advertising them for about $6/lb this week.
Ribdog buys some "better" choice,or maybe herford from fresh market for $3-$4 more/lb.
Lobel's is a major purveyor of dry aged.
Lobel's Prime Standing Rib