Dwfisk, the Top Block should be very easy to come by. I am pretty sure all butchers get top sirloin sub-primals in. Just get them to cut a thick slice off of one. If you are looking for the old style, use a full, bone-on Prime Rib. They are delicious done this way. Due to their sudden popularity, tri-tip will be the hardest to find.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."