I have broken off a blade from the fan on an MSI R6790. I tried contacting the seller and other distributors in the country (Hungary), but they couldn't help. I have received no help from the MSI customer support.
I found a fan for sale at http://www.aliexpress.com/fm-store/508904/211401938-526862961/Power-Logic-is-PLA09215B12H-Diameter8-5-Holes4-3CMGraphics-ball-fan.html, but I find $60 (including shipping) way too much for a fan.

Does someone have any idea, where could I buy a replacement fan?

I thought of somehow fixing a 6cm or 7cm rectangular fan there, but I don't think it would be very efficient.