I guess it depends on where you live. We don't have a lot of lamb in TX - but I could usually find sirloin chops at Albertsons and Tom Thumb when they put out a primal half broken down to cuts (back a couple of years ago when they still had butchers). These days - only two places I know of are Whole Foods Market (in Arlington) and Central Market (on the other side of Ft. Worth) ... and they both still have real butchers.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain