NBA basketball player/coach Isiah Thomas spells his name this way. He started his playing career in 1981, which may explain the renewed popularity of this spelling.
I always thought it was pronounced IS-eye-yah. I like it I-zay-AH too, that's how Isiah on Are You Smarter than a Canadian 5th Grader pronounces it. I think I would use it as a middle name. I hope it stays under the radar being such a nice name. Another person with the name Isiah is one of the Duggar kids.