What's a good substitute for tri tip?
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.
For what purpose? Grilling or oven roasting? If for sliced taco meat, then use flank steak, or I like the sirloin idea as well.
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.