K-- I drive 175,000 miles a year so I get around. I own a truck/trailer combo, the main gig, and also cook privately here and there.
Tri-tip is distinctive in appearance. They all pretty much look the same. Check the pics on the Cooking tab above where Chris has his recipe. They can be narrower but that's the shape. Most are a bit over 1.5 lbs to a bit under 3 lbs but can run slightly higher.
D/K-- I am less sanguine about 'butchers' (read: meat dept employees) and butchers in the east. I can't tell you how many have insisted that sirloin tip is tri-tip. Few actually have a NAMP guide at their disposal. My butcher, who cuts many of the cattle that are raised around me, I had to show directly but the suggestion to contact a local processor is a good one nevertheless. Many meat dept managers--if they look at their order sheets or contact their buyers--will find that they can order them (they are listed as tri-tip or bottom sirloin butt as D notes; though it is said that they are sometimes called 'triangle roast' I have never actually seen them called or listed this way).
K-- If you have no luck in your area but travel ever to central or eastern Mass, Westchester county or the Island, or Cleveland or Columbus bring a cooler and hit the Trader Joe's there.
Gary/Rick-- The Publix in Okee and in Ft Pierce has been stocking them periodically. Unfortunately, they cut them into steaks first--fine but I much prefer them whole. However, they do this in house so if you tell them you want them whole you should be able to get them that way. All of us get meat from the same DC in Lakeland so they've been available--at least sometimes.
I was originally told 3-4 years ago that TJ's was moving to Fla. Doubtless they are--their plans are to be fairly national--but I've been waiting. Now that they've opened two stores in Atlanta (with two more opening soon) and one outside of Raleigh, I expect they'll move to Charlotte, at least, and either at the same time or shortly thereafter to Orlando. Just a guess but that's their pattern.