Showing my ignorance of Hong Kong, but can you look on E-bay? It is an expensive source despite being an 'auction' but many varieties are available there.
Cacti are very difficult to grow from seed. If you want to try growing some of your own, I would suggest getting some cactus pads shipped to you. You can give some as food but save several (or as many as you want) to try and plant. All you have to do to get cactus pads to root is stick them about half way into a soil/sand mixture. Keep the soil somewhat moist (without over-watering) and after a few weeks the pad will develop roots. It will take a while for the original pad to start growing new pads, but this method is a lot more reliable than trying to grow a cactus from seed.