I don't think it matters mush where you buy it,more importantly is the way you handle it. Season the heck out of it. Garlic, Salt and pepper. Careful on the salt.leave most of the fat cap on and cook slowly. In the Santa Maria area we use oak wood but charcoal will work. Cook to a nice rare and remove cover with foil and let it sit to allow the juices to settle trim the fat and slice thin.
If you want great tri-tip at a restaurant, go to Wood Ranch BBQ in Agoura Hills, off the 101 at the Kanan Road exit. There is another Wood Ranch BBQ in Camarillo. They smoke it in their BBQ and then finish it on the grill. I know it is a long drive for you. I discovered it when I was outlet shopping.
Tri-tip is great.
How you handle it is more important than how you cook it/where you buy it.
You won't find any t-tip better than "choice" -- it won't happen.
Take it out of the fridge while you're having coffee. Wash it, dry it (thoroughly), and spark up the grill, later.
Lots of S & P, grill it uncovered to 125 degrees, tent with foil, let rest for 1/2 hr., slice thinly.
re: Steve Potenberg
Try Owen's Market on Roxbury Dr. and Pico Blvd. near Century City and Beverly Hills. Order the tri-tip pre-seasoned and ready for the grill. While you are there, try the filet tip. It's tender and flavorful (unusual for a filet).
I also like Whole Foods Market meats if Owens is not convenient.
Have you tried Whole Foods in the Rollings Hills shopping mall, corner of Crenshaw & PCH? They carry some prime beef and may have prime tri tip. Also, Bristol Farms in Rolling Hills Estates carries good meat. Tri tip can be tricky..it's often tough. Perhaps a good marinade would help.