Your profile says Jacksonville. Is that Florida? If so, according to a website that says state by state what insulin and syringe laws are, it says some counties require a prescription but most do not. The cheapest place to get them is probably Walmart. I had sticker shock when I got some at CVS last night as they were 3x the price of Walmart!
I wouldn't necessarily say you are doing anything wrong, some kitties take a while to regulate. I'm going on 4 and a half months with Bud and he's still not regulated. With the U100 syringes you'll be able to fine tune doses as you might have skipped over Spikes optimal dose by increasing in whole units. You might want to post in the ProZinc forum for dosing advice as there are some wonderful ladies over there who might have better insight than I do. And maybe ultimately Spike would do better on a different insulin. I wouldn't give up on ProZinc just yet, but it might be something to consider down the road.