93 Octane is very hard to find around my parts of Texas (High Plains), 90 is everywhere but 91 is pretty hard to find as well.Texas has 90 octane? I've never even heard of 90 octane. I just heard of 87, 89, 91, and 93 for the regular octanes here in the States. Why not just run 93 octane though?
Generally you can get away with a difference of two points in octane without any sort of issue. The computer can generally compensate for two octane points. So, I would get Bama to write you an updated tune. My guess is that they will send you a true 91 octane tune.