What is the US equivalent of "Joint of Sirloin"?
Any butcher can probably get you a Joint of Sirloin by request, but you will not likely find it "on the shelf".

A "Joint of Sirloin" is a rare cut in the US given the other possible uses of the sirloin.
If one looks at the T-Bone/Porterhouse cuts, where the the "T"-bone bi-sects the steak. The smaller side is the tenderloin and the larger side is sirloin. While it is not entirely uncommon for the short-loin primal to be cut to form a tenderloin roast from that side of the "joint" when that is done the sirloin is then usually (in the US) trimmed to be either "New York Strips" or "Kansas City Strips".



Here we see a section of the short loin primal, from this end only a thin layer of fat separate the tender loin from the sirloin.



And this is the trimmed joint of sirloin.



And the Bone-in version of the joint of sirloin