US:
A common murderer-for-hire is difficult to hire because they'll probably take your money and disappear. A muderer-for-hire that won't get caught is impossible to hire because they don't advertise to the common person.
If you want the full package then it'll cost you five to ten years: Commit a somewhat violent crime to avoid being sentenced to minimum security prison. You'll have lots of time. There's not much to do except be violent and learn whatever you want from other inmates and books. Choose your friends wisely.