You can buy them individually. I live in Germany and got mine (same version) from Amazon. My books look used af now, though. They cost me 9,90€ (around 10.91 USD or 7,81 GBP each:

The Dark Tower I: The Gunslinger:

• Amazon.com: http://www.amazon.com/dp/B003BVFZ46/
• Amazon.de: http://www.amazon.de/dp/1444723448/
• Amazon.co.uk: http://www.amazon.co.uk/dp/1444723448/

The Dark Tower II: The Drawing of the Three:

• * Amazon.com: http://www.amazon.com/dp/B003BVFZ6O
• * Amazon.de: http://www.amazon.de/dp/1444723456/
• * Amazon.co.uk: http://www.amazon.co.uk/dp/1444723456/

The Dark Tower III: The Waste Lands:

• Amazon.com: http://www.amazon.com/dp/B003BVFZ6Y/
• Amazon.de: /
• Amazon.co.uk: http://www.amazon.co.uk/dp/1444723464/

The Dark Tower IV: Wizard and Glass:

• Amazon.com: /
• Amazon.de: http://www.amazon.de/dp/1444723472/
• Amazon.co.uk: http://www.amazon.co.uk/dp/1444723472/

The Dark Tower V: Wolves of the Calla:

• Amazon.com: /
• Amazon.de: http://www.amazon.de/dp/1444723480/
• Amazon.co.uk: http://www.amazon.co.uk/dp/1444723480/

The Dark Tower VI: Song of Susannah:

• Amazon.com: /
• Amazon.de: http://www.amazon.de//dp/1444723499/
• Amazon.co.uk: http://www.amazon.co.uk/dp/1444723499/

The Dark Tower VII:

• Amazon.com: /
• Amazon.de: http://www.amazon.de/dp/1444723502/r
• Amazon.co.uk: http://www.amazon.co.uk/dp/1444723502/

For US readers: eBay or other book sites will do I guess.