Busse, Swamp Rat, and Scrap Yard all basically offer limited releases only. They'll usually announce new models on the forums, or introduce them at various shows (like Blade), and you'll have some limited amount of time to order one.
So once a model stops being made, it may never be offered again. There are some exceptions to this, most notably that some currently unavailable models are usually produced for the various shows. Blade & Knob Creek are probably the main shows, and usually have a good/great selection, depending on what you're after.
Jerry Busse will usually post pics a few days before a shoe to give people an idea of what will be offered. There's usually a pre-show sale at the hotel the night before, for those attending the shows. This gives people an opportunity to interact with the Busse folks, and basically hang out, eat, drink, and what not.
So if you are going to a show... you'll want to go to the pre-show sale for the best chance to get what you want. If you're not going to a show, you'll need to try and line up a proxy, which is somebody that orders or buys the knife on your behalf at the show. How the actual proxying takes place is up to the proxy. Otherwise, you can usually expect something to be offered on the exchange after the show, usually at higher than show prices.
Basic rule of thumb... pay attention to what's being offered by Busse, Swamp Rat, and Scrap Yard at any given time. If the opportunity is there to get it from the factory store or as a factory pre-order, then get it. That way, you'll get the best prices. Or try to get something from a show at show (usually factory+sales tax) prices. Lastly, hang out in the exchange & trade forums and see what comes up.
