Religious ceremonies (also called rituals, meetings, or "sings" by Indians) are private events (by invitation only). This is because they are tribal religions; they are not Christianity. Recruiting new members is not the primary focus of Native religions. Relationships in Indian communities are based on extended family relations.
If you are there, it's because you belong there due to a family relationship, or because you are a guest. It follows naturally from this that ceremonies are not advertised, and Indian religious leaders do not publicly seek converts. Indian ceremonies never, never charge admission. Sometimes they may take up a collection (like a church), but this rarely covers the transportation costs of the religious leader. It's not required, but it's a pretty good practice to bring a gift to the religious leader. Such gifts include tobacco (non-alcohol cured is best), a sacred herb, or some item which the leader would find useful, such as clothing. You can make a gift of money, but it must be in an envelope, and the religious leader is absolutely forbidden to ask for money or other reimbursement. If you want to be prayed for, bring tobacco to the leader. Ceremonies are not political meetings. You cannot bring your personal political agenda into a ceremony and pray and preach about it.
Movies: Little Big Man (starring Dustin Hoffman) tells about the beginning of the reservation period and about Indian attitudes, humor, and spirituality. Powwow Highway (starring A. Martinez) is one of the best and most balanced movies about Native spiritualities today. Another fine movie is Smoke Signals. It was in theatres late in 1998, and we can only hope it will be released on video soon.
Books: There is no one Native spirituality or culture--there are hundreds. Here are some entry points. For traditional Lakota spirituality, read The Sacred Pipe by Nicholas Black Elk. For current Native Christianity, read Native and Christian (edited by James Treat). For the Native American Church, read One Nation Under God (edited by Huston Smith and Reuben Snake).