There are a few things, if you can find a real butcher of someone at least willing to talk with you. Almost every store is getting a sirloin primal of some sort or another. Traditionally, since the 1950's, the three cuts of sirloin, that all have been used in Santa Maria style BBQ are the Top Block, The Ball-Tip and the Tri-tip, any of these done to a nice medium rare will serve you well. Of the three, Top Block or Tri-tip are what you want. A whole trimmed tri-tip will always run between 1.5 and 3 pounds, Top Block is more around 4 to 6 pounds, and Ball Tip is around 5 to 8 pounds.
See if you can score any of the three main sections from the Top Sirloin, Bottom Sirloin sub-primal, it will always be one of these three. Bearing in mind that the Top Sirloin is not the same as Sirloin, which is located above the tenderloin.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."