Not many real 'butchers around anymore, but any grocery store that sells either the 2-3" cut short ribs, or the thin cross cut Korean style - they should be able to sell you a "Full untrimmed rack of Beef short ribs in cryovac" - that is what I ask for. After the dear in the headlights stare is over, you reiterate your request a few more times you can have success.
You're looking for a 3 bone (sometimes 4) plate of short ribs. ALSO! Do not pay trimmed price for untrimmed rack of short ribs - Insist on an untrimmed price. A buck/lb off works for me.
One last thing, some Restaurant Depot have cryo of 4 racks for awesome price - if you got a crowd to feed.