There's lots of sex shops in Hong Kong.. you just need to know where to look..
There's a arcade mall which sells 1000's of new mobile phones, located near the Mong Kok East station..
the mall is very popular with local HK teenagers, etc... and usually flooded with gang members.
It's called Sin Tat Plaza, address is in 83 Argyle Street.. go up to the 2nd floor and walk around,
there's a few sex stores there, and also a small tiny shop that sells Japanese AV DVD's.
That arcade mall specialises in mobile phones, toys, and clothes.. and you get the few odd shops here and there.
If that's not good enough, you can check out other arcade malls a long Nathan Road, I saw some sex shops in those aswel..
Mong Kok has lots of places