Try going to clothes' alteration shops round your neighbourhood. I am sure they will be able to sew you some. If you don't know where these shops are, ask your dry cleaner. They will definately know which one is the closest one for ya. Else, Worldwide House in Central got tonnes of alteration shops.
Next, you will need materials, you can buy the normal sheets (non-fitted ones) from any baby shops like Mothercare, or even department stores' baby section like Jusco, Wing On etc, or beddings shops like Aussino.
If all fails, try buying non-fitted sheets fasteners (kind of like a white rubber band with some sort of pastic hook) to pull the sheets securely under the mattress. I remember seeing them in Eugene Baby Shop in Windsor House, but have not tried them, so I have no idea how safe or useful these are. Best advise I can give is to get your fitted sheets made by clothes' alteration shop (if you are positive that ready made ones can not fit) and only get those hooks thing if you are desperate.
