Here's a link that allows you to add an Echo to your cart:
http://www.amazon.com/gp/offer-listing/B00L9EOQCO.
The problem is, you can't actually
complete checkout unless you've received an invitation (or are about to get one, perhaps).
After pressing the "Proceed to checkout" button, you'll see the message "We're sorry, purchasing this product is by invitation only."