I have never seen a brisket at CostCo in the SF Bay Area. Smart 'n' Final has them, though they only carry Select grade beef, which I don't care for. I don't know where you live, but you can get Choice whole packers at Dittmer's in Mountain View. If you aren't near there, I'd suggest finding a local butcher shop. If you are anywhere near a Lunardi's, you could probably get one there, too, though they are very pricey.
Good luck!
Rich
PS: $5.00/lb is a price for TRIMMED brisket more than likely. That price factors in the labor. The Dittmer's price is $2.79/lb the last time I bought one.