It depends on the skier, the brand and the sports store.
If you're talking about the very top racers, who win or contend for crystal globes, they may have highly customized skis built for them.
Lower-level World Cup racers just have ordinary race stock skis. There may be multiple versions of a particular model of race stock ski, though they all look the same cosmetically. These generally are not on sale in run-of-the-mill sports shops, but are available from reps. Depending on the brand, they may be available on the rack at some shops or (more commonly) some shops may be able to order them for buyers. Exactly who reps will sell to, and whose orders they will place through shops, also depends on the brand. In some cases, it's going to be limited to people who meet certain standards, in others it might be open to all USSA racers, and in others they'll sell to anyone.
Race stock skis also turn up on eBay, at some online retailers, in race-club classified ads and other random places from time to time. Sometimes they're unusued skis with old cosmetics, sometimes they're used (perhaps only slightly used) skis that a racer has no further use for. While you're unlikely to find Lindsey Vonn selling extra skis on eBay, if you looked at the right auction at the right time a few years ago, you could pick up some from her now husband.
When it comes to speed skis, the story's a bit different. The best DH skis are most definitely reserved for the very best racers. In some cases, the basic ski may be years old, and may have "belonged" to some other racer previously. All speed skis are essential race stock, since they're not a consumer product at all.
These skis aren't generally available because there's really no good reason they should be. A GS ski designed for a top level racer, for example, is designed to do things (including complying with rules, as well as making turns of a type and at speeds) that has little to do with what even an expert free skier does. While there's probably some disagreement here, I'd also argue that they're not really the right ski for a more casual (ordinary Junior, Masters or beer league) racer either.