I would suggest to advertise in the International Mining Journal and perhaps also the treasure magazines like Western and Eastern Treasures, Lost Treasure etc, looking for investors. As for the actual contract, the first thing would be a signed non-disclosure agreement before you disclose any kind of details. Then all you really need is a grubstake contract - the investor(s) put up the financing, you do the finding and recovery, they get half of the proceeds. Some states have a legal requirement for a grubstake agreement that has a time limit which is a good idea, I think Colorado requires a one year period from the date of signing, others might be slightly different. It is not common today but was once very common practice to have grubstake partners financing the search for minerals.
I have had grubstake partners in the past and never had any issues, but would recommend that you should be 100% honest with your grubstake partner right up front - if you have a prison record or had a bankruptcy etc just tell them up front. If that drives away your partner don't fret over it, most people are adult enough to know that everyone makes some mistakes in life. If you lie about even little things, it will raise suspicions about you in the big things and you could end up very sorry you ever entered into any partnership at all. If you can possibly find and recover your treasure without any partners involved it is the very best way, because partnerships rarely ever work out, most end up as enemies. So think hard about it before you put everything on the line for a prospective partner, because things can go very badly and very fast, you need to have NO doubts about your partners before you commit.
Good luck in your quest amigo, I hope you find the treasures that you seek.
Oroblanco