I can't find any christening gown in HK. For my kids, I ordered them from online which have many choices and the prices are reasonable. Apart from online, you may get it from nearby city as well e.g. Macau, Taipei & Tokyo.
I got mine from overseas too - so much cheaper than HK - I actually have an unused one - it's a modern style dress for a girl in a cream colour from Next. Size 6-9 months. If anyone's interested I can send a picture.