There are the general cannon curse marks that are allowed. A fuuinjutsu user can create their own though they cannot have a second stage and will be weaker than cannon ones or ones created by admin. To obtain a curse seal, you must be a missing nin. Once you pm a head admin they will run a test for you ONCE. You can attempt to get the seal at anytime you want, as little as gennin. Understand that leaving a village is not easy, and it is possible to be stopped and killed. Understand that anbu will come for you as well. The higher your rank the less chances you have of failure. If you fail your character dies, no exceptions, thats the risk you get. Also you are run by a selected person who distributes them, and to ensure so they can kill you using the curse seal. (within reason) The only way to stop this is too seal the curse seal, thus sealing its power, and maybe waiting until they die, but then having is kind of pointless them isn't it.