My Identifiers, Bowker runs the ISBN's in the US. And their pricing is a bit crazy if you ask me!
https://www.myidentifiers.com
I know there are resellers of ISBN's that supposedly sell them for less, but I do not know if they are actually legit and how to tell the difference.
I ended up biting the bullet and buying my ISBN's in bulk. I think your best option is probably going through Smashwords, $10 is reasonable and they are at least known to be reliable.
Amy