Your chances of finding once fired .300 Savage are decidedly slim. However, $34.99 will buy you a box of 50 BNIB Hornady cases from Midway. (Currently on sale. Regular price is $4 more.)
Grafs want $39.99 per 50 for Hornady. $18.99 per 20 for Jameson brass.
Like mehavey says, it's makeable without much fuss out of .308. I think, it's just a size and trim thing. http://stevespages.com/jpg/cd300savage.jpg http://stevespages.com/jpg/cd308winchester.jpg
I have both a 300 savage and a 308. I don't have many 308 brass to resize to 300 savage without shorting my 308. Also with having both calipers I would notbe that big on having 300 savage head stamped 308 for fear of a mix up.
Check in with www.oncefiredbrass.com. They always say that if is not listed, they will find it for you. And sometimes they do have unusual brass listed.
I would just buy new brass. I have bought used brass already, and usually ended up tossing a few after inspecting them. The price of used brass is usually not that great on civilian ammo anyway. Do what you want.
I have been buying factory ammo and brass ..I have been experimenting to see what the rifle likes. I usually try to have 300 cases for each rifle FWIW.