Look for heritage breeds. Most modern breeds are based from Landrace because they have the long barrel structure. Look from some of the older breeds.
Same goes for chicken. Commercially mass produced chicken has nothing on a smaller production heritage stock. Commercial mass production is the reason why everything tastes like chicken… ie) minimal flavour.
Also keep in mind that the meat in a full pork loin is not the same from end to end. For roast (SV or otherwise) I prefer to cut from the blade end, or the center as it has more internal fat and will have more flavor and stay jucier. The loin end is very lean I won’t use that for a roast unless brined. I usually use the loin end to pound flat for schnitzel.