Seat confirmation

Hi guys. I have a question about the seat confirmation how I can get a seat number if most of the airlines don't let you cheak in online 48 hrs before?. I all ready tryed to get 30 days visa from hong Kong but I was not allowed because I had to stay 30 days in Hong Kong before I can apply so now i will like to stop for few this to see the great wall . Thx for your help.