One letter domains are about as hard to get as a one letter stock ticker symbol. A few companies, such as Agilent Technologies Inc. (A), Citigroup, Inc. (C), Realty Income Corp. (O), and United States Steel Corp. (X) were lucky enough to get their stock symbols.
But in terms of getting a one letter, or for that matter, a one number domain, it is nearly impossible and very expensive to get one. One example is x.com which is owned by Paypal.
But now is your chance. Sedo is now auctioning off a whole series of one character domains including 1.biz, a.biz, m.biz, x.biz, y.biz, and z.biz. Grab 'em while their hot.