You may be tempted to buy a nice, shiny new max-level character from a website. Blizzard does not look favorably upon this action and will usually ban an account if such activity is discovered.
There are other reasons that you might want to avoid such a decision. For one, there is little guarantee that the character you end up with is the character advertised (there's no way for you to know!). For another, accounts are linked by passwords and 'secret questions', so you have to have faith that the person selling you the character is going to give you both of those. These are two of the most obvious reasons, but I'm sure other community members will provide more!
But what if you really want a near-max level character now? What if your friend is at max and really wants you to play? Well, Blizzard does have two options that players may consider: purchasing Warlords of Draenor comes with a single free character boost to level 90; you may purchase additional boosts for other characters via their online store.