Argan Oil is the bomb.com. The brand I use is One n' Only Argan Oil.
This is the only product you need to use on your Virgin/Remy Hair Extensions. It leaves your hair smooth and frizz free. I previously used John Frieda's Frizz ease on my extensions but it did nothing for me and my extensions would still get frizzy. Argan Oil also works great my own hair. I love finding products that work effectively on my extensions and my real hair. This is definitely a product that does. The bottle is small but it lasts long. A little goes a long way and I use a dime size each time I use it.
I bought mine at Sally's Beauty Supply for $13.19 CAD (excluding tax). You can find it online on Ebay or Amazon.