Unfortunately that is the experience you are going to get at Sam's or Costco. They don't really have a butcher or meat department you can talk to. They just have what they have.
This must be a regional thing. My local Sam's has a full-on butcher shop, and will gladly do custom cuts, or find things in the back for you. I buy full tri-tips there that are very good (USDA Choice), and are already well-trimmed (which means I'm not paying for a pound of fat that I'm trimming off later). Our briskets are Choice, at a minimum, and occasionally Prime. Must be a store-by-store thing, because I couldn't be happier with the service I get at mine. No Costcos here, in the heart of WalMart land, so I can't comment on them, though.