If you have a Trader Joes they usually have tri tip's from Australia in their meat section. I think the ones in the blue label are plain, they sell others that are in a marinate. I've used one that was in a marinate and didn't like it that much the plain one that I seasoned myself came out better.
If you have a local butcher or meat market ask them for whole brisket, they should be able to get one for you. I've seen whole brisket at super Walmart's, but haven't used one. Also pretty sure if you go to the meat market at the big chains, they should have them or be able to get one for you. I've seen whole brisket at Giant and Wegman's
Lots of folks on the forum get whole briskets from Costco or Sam's as well.
Tri tips are tough to get on the East coast because they are not a popular cut. Treuth's near Baltimore have them in their retail store. Just talked to them today and was told they always have them. I have tried to order tri tip from chains, local butcher and meat market, but they typically will only sell by the case because they can't move them.