I am not aware of that Sams has buttercream icing you can buy? Is that true and where do you find that at Sams? Do you mean Sams store where you have to be a membership?
Yes, the Sam's Club where you have to be a member. You can buy buckets of their frosting - you have to ask at the bakery counter, it's not out on the shelves. At my Sam's you have to put in an order and pick it up later that week, others here have said at their Sam's they could just go and buy it without an order.