Bookstores here have a rather limited selection. They're also incredibly expensive, if you're buying fresh pages. Getting books online is your best bet, both in terms of prices and options (just make sure that your shipping charges are fair, though).
Here's a short list of places where you can happily browse and binge buy without feeling too guilty. They also offer great discounts!
Book Depository
This is my personal favourite because they offer free shipping, and because the prices aren't too bad. There's a Bargain Section too, which is pretty addictive if you're a hopeless case who likes binging on books but is also broke. They generally have a lot of discounts and sales, and you're conveniently notified of these if you've subscribed to their (actually not annoying) newsletter.
World of Books
You can find used books here, in good condition and for cheap. Prices are in pounds and not dollars, just in case you miscalculate the conversion rate. Books sold in bundles are super cheap, with complete trilogies going for just under 5 pounds.
BetterWorldBooks
Featuring a massive selection in new and used books, these guys offer a bit more than your average online store: you can sell, donate, and buy gift vouchers. Like the site says, Nothing says I love you like a few lazy clicks of a mouse. A BetterWorldBook.com gift certificate is a great way to show your favorite reader that you care (with very little effort). They have a tab for rare and collectable tomes, and for audiobooks as well.
Amazon
This is one of your more obvious online shopping options. The site's rather clinical, especially when compared to Book Depository or BetterWorldBooks, but it's also organized a bit more tightly; like for instance, you can browse for newly published books, and also upcoming books (on the far left column). Their Kindle eBooks Store is a lot nicer.
You can buy books online through some of the local bookstores too (Makeen, VijithaYapa and the likes), but it's a bit pointless when you can just walk in and get the same thing anyway.
If there are any reliable and cool stores we've missed, do let us know. Happy reading!