I don't know about IBA specifically, but in my experience making tissue
culture solutions most plant hormones can be dissolved by adding to small
amounts of either:
In Dodds and Roberts "Experiments in Plant Tissue Culture", they recommend
dissolving the indole auxins in 1 N NaOH. If you find you still have
undissolved material, perhaps it is the non-hormone material in your stock
(you said it was 99% IBA). If it seems like virtually all of the material
you add has not dissolved, you can also try warming gently. The non-indole
auxins, like 2,4-D, can be dissolved in a little ethanol, then diluted.
Dodds and Roberts recommend dissolving the non-indole auxins in DMSO, but
you have to be careful with that stuff because it penetrates the skin so
easily. I guess you could try that if all else fails.