When at Frank's I select the slats according to straightness, low crown, low nodes, distance of nodes, and clean rind, no scratches.
If you buy a bundle, you get whatever is in the bundle, if you buy 5 pieces online, you get whichever five are easiest to grab.
Every time I go, I tell them what I'm doing with the slats and explain which are good and bad for my purposes. I don't think they care, most of their slat sales go to tiki bars and such.
1 1/2" is plenty wide for osage bows, ipe bows are even narrower. If the bamboo is selected properly, it won't have much of a crown, the high crown stuff is what requires wide widths and more waste.