Well, if you can post on here, you can buy online. The challenge is having a means to pay for it. Check with your bank about buying gift credit cards. If you have the money to buy the book, you can get one. You may need your parent's permission. You pay the bank, and order online with the card. Easy as pie.
As for finding the sales codes, that's a bit tougher to find. What Kalmbach needs to do is make it easier to find their products on their own websites! This took a bit of drilling through multiple pages before I hit the right one!
General books on scale modeling:
http://kalmbachcatalog.stores.yahoo.net/scale-modeling-books-general.html
Military books:
http://kalmbachcatalog.stores.yahoo.net/scale-modeling-books-military.html
The code for the books are the five digit numbers to the left of the price. For instance: Shep Paine's "How to Build Dioramas" is 12136 for $24.95 + S&H. Take that to your LHS and see what they can do for you. You will likely end up paying more than if you figured out a way of doing it online.
As for distributing their books worldwide, that's up to Kalmbach to answer, but I would imagine that the costs of distribution would drive the price of the books higher than they are worth. Very hard to justify a $40 price for a book you can order online for $20.