Good brass
Enron Exec--As pointed out, you ain't gonna find a lot of .300 Win Mag brass lying around at the range. It's pricey, so people who reload will snap it up.
Also as pointed out, anything but A-merc is usable brass to reload. (Don't bother with steel or aluminum cases however!) I've never seen A-merc brass in anything but pistol calibers. Any of the "big names" in American ammo use good brass, rifle or pistol, which reloads just fine.
Oh, and watch out for Berdan-primed brass, also. It's reloadable, but only with special equipment and special primers. It has 2 small flash holes instead of one larger central flash hole. Don't think there is any Berdan primed .300 Win Mag anyway. If you find any Berdan-primed cases, they make dandy scrap brass.
For pistol reloading, the quality name is Starline brass. I can't imagine why Jeepmor is having a bad time with it--Starline generally enjoys an XLNT reputation.
For rifle reloading, the Cadillac brands of brass are Lapua and Norma. Nosler Bullets says they put out brass now, of high quality, but no experience with it.
Top-grade brass will be more uniform in metallurgy, weight and thickness, as well as having the flash holes drilled rather than punched. Makes more uniformity in yr loads, with less case prep. If you're after the finest accuracy, the extra expense is worth it. Or just if you're fussy about your brass.
If you're less fussy, any of the major manufacturers' brass will do just fine. The more "into" reloading you get, the more fiddling around you will do with yr brass. But as a starting reloader, get the basics down first before you begin sorting brass by weight, reaming primer pockets, turning necks, etc, etc.
That's one of the neat aspects of reloading--You can make cheap, good, decently accurate, hunting ammo and be perfectly happy with that, or you can go to all sorts of extra work and hassle to wring the ultimate in accuracy and/or velocity out of yr loads, or anything in between.
Just follow the recipes in the loading manuals, don't get creative, play safe, and it is a lifetime pursuit. Enjoy.