You're not going to have many options. The Savage 99 was a standard hunting rifle in the decades when steel and walnut were the only choices. Composite, injection or fibreglass stocks didn't really come along until well after the 99 went out of production. Wikipedia puts that end date at 1998, but I doubt the line was making more than a handful at a time.
In your situation, I suggest you need to get a stockmaker with a duplicator to machine you one from a blank or modify something you find that you like. Because it is a two piece design, you could use pieces from a Winchester lever (if there is even a plastic model in the aftermarket), or a Lee Enfield No.1 or No.4. But, be prepared to cut and fill voids and seams with epoxy or JB Weld.
Good luck!