Well, now that no one cares why they haven't seen Tri-tip roasts in their local grocery, I can post the alternate names you may see instead without being concerned that I'm upsetting the balance of the thread by sticking to a TOPIC.
Beef tri-tip:
Other Names: Whole: Bottom sirloin butt, corner cut, knuckle cap, punta en triángulo (Spanish), Santa Maria barbecue, sirloin triangle muscle, triangle roast, triangle tip. Steaks: Coulotte steak, triangle steak.
However, I've never seen any of these names. It's part of the sirloin or round. It's popular in California.
And there's a picture here:
http://www.chow.com/ingredients/257 and lots more information.