Obviously, every player is available for the right price. Ian Desmond is one of the top shortstops in baseball, so we know the price would be steep. The Nationals recently swapped former Yankee farmhand Tyler Clippard (hey, the Jonathan Albaledejo trade!) for former Rays shortstop Yunel Escobar (who has now been traded twice in a week).
All of baseball knows that the Yankees' farm system has gotten better with strong player performances in 2014 and with the arrival of several prospects via trade this winter. They've hesitated to trade for Cole Hamels because his premium salary negates some of his value. Ian Desmond isn't quite as valuable as Cole Hamels, but the difference is probably not as big as you'd think. A rumored three-way trade between the Mets, Rays and Nationals nearly sent Desmond to Flushing in exchange for the Mets' #1 prospect. (Interesting that the Mets' and Rays' ownerships also recently sided with the Nationals in their dispute with the Orioles. Totally not suggesting that there might be a connection between the two events. That would be crazy, wouldn't it?).
So, the answer is probably that Desmond, a free agent after the 2015 season at 30 years old, is not available at a price the Yankees front office will consider. But I was really thinking about Escobar. As a righty with a track record of decent defense, coming off a down year, under team control for the next three seasons, Escobar would seem like a more natural platoon partner for Sir Didi Gregorius, and a more impressive backup at second base and shortstop than Brendan Ryan. He represents a chance for a rebound season, for three years of team control (if his $7M 2017 team option gets picked up), and for the kind of incremental upgrade that Brian Cashman has obviously pursued over the last couple of years. He hit .270/.347/.342 against lefties last season, much better than Derek Jeter. Better than JD Drew, too, but you knew that already.
Could the Yankees package Ryan with a reliever for Escobar (pushing Danny Espinosa into a battle with Anthony Rendon for the 2B starting role)? Is it enough of an upgrade for both teams to make it worth the risk? I doubt it. What if the Yankees include another few million dollars? The Nationals have been vocal in the past that their payroll is "tapped out" at its current level. What if the Yankees include one of their young second basemen, either Rob Refsnyder (who's not on the 40-man and would demand a roster move before he could make the team anyway) or Jose Pirela (who's more versatile defensively but more limited offensively)? For the Yankees, would Escobar (87 OPS+ over the last three seasons and already 32 years old) as a backup at shortstop, third base and potential starting second baseman represent an improvement over Brendan Ryan and Pirela/Refsnyder?
In the end, I think if the Yankees are interested in anyone currently on the Nationals' roster, they'll wait for Desmond to become a free agent in October. But you'd have to think it's worth kicking the tires, right?