Tri-tip is from the sirloin primal, it was traditionally a waste cut from the butchering process due to it's odd shape and strange grain pattern. Since it was often removed at the initial butchering stage, the cut was very inexpensive, so the local farmers, ranchers and town folks in the area that the cattle were intially rail headed and broken to wholesale primals were able to buy it cheap.
There's a Trader Joe's in Brentwood, MO., outside St. Louis. They have great tri-tips. They are out of Australia. Make sure you get the non-marinated one, so you can do what you want with it rubwise.
I'm making an assumption about TJ's in Brentwood carrying them. They carry them down here in Atlanta. A quick call to them should confirm it for you.