If a product is in stock you will see Availability: In Stock on the top section of the product's detail page. Products that are "In Stock" usually require 1-2 business days processing time before they are shipped out. If a product requires more processing time it will be noted.
When a product goes out of stock a "Backorder" button will show up in place of the "Add to Cart" button. In addition, a time frame such as "Usually ships in 2 days" will replace the in availability field. The time frame denotes when we expect the product to be back in stock. Until the product is back in stock and the "Add to Cart" button reposted, the item cannot be ordered.