Is there a form for such an agreement? where can I get it?

No, you should consult with an estate planning attorney to have one created. Best wishes!

The agreement can provide that you keep the property in the event of a divorce. Generally speaking, to be enforceable you and your spouse will need separate attorneys. That is because to be enforceable, you will need to demonstrate your spouse understood it, and the easiest way to do that is for each spouse's attorney to verify in writing that the attorney explained the agreement, and its consequences prior to signing. Courts have refused to uphold agreements like this when a spouse who had no attorney advising them before signing later claimed they did not understand all the rights they were giving up by signing.

To clarify, I am referring to a post-nuptial agreement, which is similar to a pre-nuptial agreement, and not a simple community/separate property agreement. In any event, as mentioned in my previous comment, you almost certainly need a lawyer's help with this.