I bought my AF Ones in Gigasports in Tai Koo Shing in December last year. I had to search everywhere for the black ones and Gigasports only had a few pairs left. They seem really popular.
Are you looking for any specific color? Purple, white and neon green don't seem really popular (at least a few months ago) and most sports shop had them in stock.
However, if you're looking for a specific color, you may want to try a shop which specializes in these "cool" shoes. It's located in a mall near Sogo which is really popular with teenagers. I remember seeing AF1s there. I can't remember the name but I think it's something like "Causeway Bay Place".
Good luck on your search!