I got the package of number 66 bobbins at walmart for 97 cents, and they had both the metal and plastic. From what I've heard, you want the type that came with your machine, which in this case is metal. I'd gotten some earlier from a sewing machine repair man, and he charged me $1.00 each!!! But that was before I knew what size my machine needed.
To determine what side your machine is threaded from, look at the last little loop by the needle that the thread needs to go through. That will always be on the side where the needle is threaded from.