The best thing to do is search around the internet for designs that you like, try looking up people like Leo Zulueta (although there are loads of others who do a lot of tribal stuff), and then take some designs to an artist who’s work you like and see if they can come up with something. Also, I’m asuming by tribal you mean the “modern” tribal rather than the more traditional stuff, but if you want the latter (maori etc) then may be worth heading to a library – or again, trawling the net! Good luck:)