I really like tri-tip because it comes from the bottom sirloin which happens to be one of my favorite primal cuts for steaks. But, here in NE Ohio Tri-tip is hard to find, and when I do find it, it's fairly pricey. I can actually get rib steaks or New York Strip loins for the same price or less.
Tri-tip is the triangular-shaped muscle on the very bottom of the bottom sirloin. Depending on the size of the cow, it will range in weight from about 1.5 - 3 lbs. Sometimes it's trimmed with the fat cap still on but usually the fat cap is removed.
If you can't find it, you can always ask for some thick cut bottom sirloins and you will end up with a very similar product in taste and quality.