May I ask why you feel the need to visit a reservation specifically? I'm not saying it's impossible, it's just a matter requiring quite a lot of diplomacy. It's definitely NOT a "better to ask forgiveness than permission" sort of scenario. Put yourself in their shoes-- european-descended people are responsible for untold suffering for your people, and have assured you that this tiny bit of land is yours, and inviolate. But here comes a guy who figures he can go wherever he wants. It might even be worse if you start flashing money around. It's a story many people you'd be encountering would find all too familiar.
The best way to find yourself on (non-casino) reservation land is to make a human connection with someone who lives there, and express genuine respect and curiosity about the culture. Probably the worst way is to try to "wander in" or buy your way in.
No offense intended to you at all, just trying to highlight some very sensitive and painful cultural issues that require one to tread very carefully.
Edit to add: There *are* a number of reservations that welcome visitors, but most of those restrict access to certain areas. Be careful to respect the boundaries that are set, and remember that for all intents and purposes, you are in another country, and should act accordingly.