I have access to a nearly endless supply of once fired 308 brass, which is great except I don't have a .308 rifle. How tricky is it to form it into 300 Savage? Would I have to turn the necks?

Why not try it and see if you can?
It is easy to take out the expander ball, lube up the outside of the 308 case, and size it with a 300S die.
You are going to have to trim .131" off the case mouth length.
308 case necks are between .3415 and .3435" in diameter.
300S chamber necks are between .3423 and .3443".
This would suggest that ~ one in nine~ .308 cases will not fit when seated with .308 bullet.
But actual case dimensions are almost always at the small end of the allowable SAAMI range, so you will likely never have to turn necks.

Hey Belaw, Tnekkcc is right, you can do it.

I like to use Imperial Sizing Die Wax which I believe is now sold by RCBS for all case forming. But any Lube will do for this conversion.

And he is also correct about needing to trim a good bit off the Necks. I wore out a couple of Lee Cutters doing just that, but they are replaceable and inexpensive.

If you are doing this for a M99, I'll recommend buying some "Once Fired Military Cases" that have been deprimed and the crimp removed. They will normally be a bit stronger in the Pressure Ring area. This will allow you to normally get 3-4 firings without a Head Separation in the M99s, if you keep the Loads at 300Sav levels.

Any M99 really needs to have the cases checked for Incipient Casehead Separation with the old bent wire trick. Just cut a wire (Paper Clip, etc.) off with a pair of Side Cutters so you have a chisel edge. Then bend the wire into an " L " shape and use it to feel inside the case for the "groove" right at the Pressure Ring. Once you do it, you will know what I'm talking about.

You can also Section a case to see where it is. Or, if you have a Speer manual, let me know which one it is and I can tell you which page to look at to see it.

Best of luck to you.