Tribal courts generally have a separate admission requirement. Check with the tribal court to see if there is alist of lawyers who are admitted to practice before that court.
Contact the Tribes Human Resources or TERO Office. Get a copy of their employment handbook and a copy of their employment laws, then contact an attorney practicing near the reservation. If they can't help you, I'm sure they will know of an attorney that practices in the Tribes' court or can become admitted to practice if they feel you have an actionable case. If no local attorneys are willing to assist, call the tribal court clerk and ask if there are any attorneys admitted to practice in the tribal court that have handled employment issues in the past. You could even contact the Tribes in-house counsel to see if they know of other attorneys that have handled employment law matters in the tribal court.
Every Tribe is different. Some tribes have an administrative process for employment grievances. Others have promulgated an ordinance that grants a limited waiver of sovereign immunity and allows their courts to hear these kinds of cases.
I am a member of a number of tribal bars in Washington State and have handled these kinds of cases before.