I live in Tri Tip country. On the East coast it might go under the name of triangle steak. Typically a tri tip weighs about 2 1/2 pounds. You cook it over real wood with minimal seasoning (salt and pepper). So to sub, look for a cut that is marbled and meets that size. Flank steak might work but it lacks the fattiness.

For what purpose? Grilling or oven roasting? If for sliced taco meat, then use flank steak, or I like the sirloin idea as well.

A fatty sirloin may work . The Tri tip is marbled,usually,so , look for some thick sirloin cut with fat to keep it from drying out. Cooking it rare , too , helps to retain some moisture and flavor.