Every place I know of that has US lamb, sells USDA Choice.
WE used to purchase a whole lamb at the 4-H auction but these are graded on the hoof and never quite reached Choice in my opinion.
But we did get to go to the 4-H Appreciation dinner, every year before ‘farm to table’ became a catch phrase.
I purchase from Fresh Farms now and once a year a ‘Baby’ lamb.
The New Zeland lamb I purchase from Sam’s is more akin to mild venison regarding the fat content.
When I was able to source USDA Prime lamb, it was the best I have ever had.
If someone comes up with a source, I would certainly be willing to try some.
We eat a lot of lamb especially in Indian/Pakistan/Middle East/Moroccan cuisines.