Check Amazon, you can find lots of the mattresses you'd find in Macy's or Sleepy's at great prices (3rd party sellers).
Being that its a 3rd party seller, you can try the following to get a better deal: Figure out who the seller is and contact them directly and ask for pricing on a specific model. Compare that with Amazon and if its the same, ask for a discount since they don't need to pay Amazon fees if you purchase from them directly.