I've never had any problems finding LH rifle's.But you can shoot a RH just the same,I have done it my entire life.
I had a Ruger #1 300WM for a long time,they are great guns.That said,the #1's either shoot good,or they shoot bad.But they will all shoot minute of deer or elk.
Gunbroker has several LH rifles in 300WM and also 300 RUM,300 WSM if you are set on a 300.
A 30/06 or 7MM Mag will suit your needs also,and there are plenty LH rifles in those calibers for sale on Gunbroker also.
If your local gun shop won't order you a rifle,I'd find another gun shop to deal with.
Savage,Remington,Tikka,Browning,Ruger all manufacture LH rifles,and their distributers usually have them in stock,so getting one to a dealer shouldn't be a problem.
Oh,CZ-USA also makes lefties!