I too live in California and tri-tip are as common as ground beef. Lots of places, including some great specialty stores, sell pre-marinated and/or pre-seasoned tri-tip. My experience is that even when I purchase from places that have the best meats, or from places that also sell smoked products, the pre-marinated products are absolutely awful and even the pre-seasoned are not great.
1. They sit in the spices too long.
2. No one has rubs that are as good as what you can make yourself.
Since applying a rub takes only a minute, and even marinating is quick, there really isn't much reason to purchase anything other than the bare meat.
However, since you can only get the marinated version of tri-tip, perhaps you should look at this page which provides alternative names for the same cut:
12 Other Names for Tri Tip
Perhaps you really have it, but it is packaged under a different name. If you do manage to find some, I recommend either Jeff's original rub or Jeff's Texas rub. The recipes for both can be purchased directly from Jeff.