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My Sam's went nearly all flats a few years ago. For home use a big flat will work fine. Since I'm doing more pastrami and such the flat is easier to prep.
I get full packers at WalMart SuperCenter or GFS. If necessary I order from GFS by the case.
Depending on what's in your area check out Restaurant Depot, BJ's, Costco, etc.
I have always been able to get some large packers at our Sam's. Now if you are sure that they dont have the big packers maybe ask before you leave. Then I would go to either Restaurant depot or even Walmart has some packer briskets.