If you just want the 300 RUM to size up for a 338 Edge, you can buy head stamped Bertram 338 Edge brass from Defensive Edge. Not cheap but supposed to be good brass.
Ultra mag brass is hard to find but is available at a cost. If going with a 338 caliber 340 weatherby brass is available in a good supply it seems and is cheaper than ultra mag brass. Plus it will give the other 338 cals a run for their money performance wise too. Its worth a consideration.
This is the finest brass available for the 338 Edge, produced in Australia by Bertram it will last 5 times longer than standard Remington Brass. Weight runout is 1/2 what Remington brass is and neck thickness is extrememly uniform, Extractor grooves are machined. This quality of brass is a serious bonus for the 338 Edge shooter.