Sometimes the discontinued model shows up in the Clearance section of the online Apple Store but one can never count on that happening. Apple manages their own retail availability very tightly so the clearance models typically sell out quickly.
Their channel partners (i.e., third party retailers like Amazon, B&H Photo, etc.) sometimes have a longer window of availability. Again, there is no guarantee. After all it's a discontinued product.
The old one is often available for a while as a refurbished product as well from Apple's own online store. Again availability cannot be expected.