I used to go to National Bookstore here. Is there another store here that sells best sellers? Can you download?
Try Amazon. I've ordered books from them and was happy with them. Delivery time was a little over a week.
There is a shop in Robinsons beside the foodcourt and between the travel agent and toilet that sells books, don't know if they have best sellers.
I've bought countless books from the bookstore in Robinson Mall. You have to search a long time for what you are interested in though, as it requires moving piles of books around.