From 9to5Toys.com:

We reported last week that Apple’s new lust-worthy 5K Retina 27-inch iMac was up for sale at MacMall with a small discount and tax advantages in most states. Over the weekend, many more retailers began offering the new product. Among the biggest:

At the moment, MacMall appears to be a best bet when buying either (and they are already shipping), but numbers fluctuate. Keep an eye on 9to5mac.com/products for the latest best prices on Apple products or 9to5toys for the best prices on accesories. .