Can you get a pre-nup after the marriage ceremony?

You can certainly obtain an agreement establishing property, alimony, and other rights in the event of divorce, even though you are already married.  However, it would not be called a "pre-nuptial" agreement, but instead the type of agreement you are seeking is called an "ante-nuptial" agreement.  It is essentially the same as a PRE-nup, except that it is formed AFTER the marriage.  In other words, it has the same substantive effect (if done properly, and for this you should search for and speak with experienced divorce lawyers), it is just given a different name.

Many states have case law requiring detailed disclosure among the parties concerning each other's finances in order for the agreement to be valid.  So, again, to be safe you should retain an attorney to draft the agreement.