'sorted'** as in 'It's all sorted" means all arranged, all in hand, fixed up.
whizz is slang for 'speed' = amphetamine, sold illegally, particularly in clubs.
So, if some is 'sorted for whizz', they have purchased and have their supply of whizz for the evening
if asking, "Are you sorted for whizz?", he is asking if they have all the whizz they want, because he, the speaker, is selling...or wants to ask him to share what he has!
** I hadn't heard this use of the word until I arrived in Britain after many, many years overseas. I was used to Australians saying, "No problems", meaning everything was in hand, or could he arranged, done, managed with ease. "All sorted', meaning 'all arranged', sounds as if it came from a Royal Mail analogy, and mail being all sorted and so ready for delivery. Someone who knows the background to the emergence of this now ubiquitous saying might add some hard fact to my supposition.