Because there is no authentic tribal dance there is also no authentic tribal costume. Many people choose to create their own costume based on what they believe tribal dancers may have worn. They are typically a one piece costume instead of the standard two piece belly dancing costume.
The tribes of the ancient cultures inspire the costumes. This type of inspiration is particularly appropriate for tribal dance costumes. It allows the dancer to wear a costume that she deems appropriate and feels comfortable enough to dance in.
Is it Authentic?
There is very little information on the origins of belly dancing. This is why there really can be no true answer to the authenticity of tribal belly dancing. There may very well have been a time when tribal dancing was practiced, but there simply is no record of it in history.
What we do have with tribal dancing is a dance that has been influenced by many different styles. Dancers will take aspects of Turkish dance as well as Moroccan, gypsy and anything else they would like to adopt in their routine.
Tribal dancing is usually a very fast paced dance. The dances of Egypt and Turkey are a slower and more sensual performance. Tribal dance is frenetic and energetic. There can also be singing and clapping added to the dance routine if the dancer feels inspired.
The music for tribal dance is usually in keeping with the style. You won't find much use of the classic instruments in tribal routines. This style of dance is a free expression for the dancer. She is able to add elements at her whim and dance how she feels. A dancer can do some research on the ancient forms of dance, but she is still able to incorporate modern tribal movements and add them to the old.