Ex junkie here.
I agree that around the festival venue there's gonna be A LOT of cops, they do so every year. I am from Split actually.
You can find drugs in Split easy and it's not too dangerous for cops(only danger is being ripped off-don't pay until you see the gear in front of you). You just try to talk with locals around the town(young guys with druggie looks - thin, not muscular... or ravers).
But anyway, LSD is harder to find so you can buy those in Amsterdam(they're easy to smuggle too), but in Split there's plenty of X/WEED.
I would avoid buying on/around the stadium, cause it's very dangerous on festival days, better to try and buy somewhere else around town.