It should be in the bakery department. At both my Sams stores, they usually have buckets of the regular butter cream icing just sitting out, but you can ask for the bettercream. It comes in prewhipped buckets...I think it's 28lbs...it's a lot, but you keep it frozen and just scoop out what you need each time.
Basically, anything Sams uses in their bakery, you should be able to buy. I like shopping there, but honestly, they aren't the sharpest of folks sometimes. I buy my icing sheets there and yet they told another cc'er at the same exact store that they didn't sell them...less than 3 days after I bought mine.
She had to go back and give them the upc numbers I emailed her to prove to them they did and she got them with no problems after that.
