There seems to be quite a bit of animosity in this thread. To the original poster, unless you find some old Remington reduced ammo, you will have a hard time finding anything like you are looking for.
The comments on getting someone to reload for you if you don't yourself, is probably the best start. Yes a good recoil pad will make a huge difference, and so would a brake, but if your muzzle isn't already threaded, it becomes a pain.
Everyone hating the poster for having a 300win mag and wanting reduced loads is awfully rude. You don't know his exact situation, and if you do end up reloading it's a great choice for a single rifle that does it all.
There is quite a bit of reload information for reduced loads, even in the Hodgdon website for 150 grain bullets. H4895 or Accurate 5744 are the best choice for a good clean burn and accuracy, rather than running a really slow powder. Trail boss as stated is a excellent powder, and would be for very reduced loads that are extremely fun to shoot.
Wish you the best of luck on finding factory reduced loads or having someone make some for you. The more trigger time you get behind the rifle with the reduced loads you will be able to work your way up. And if you are still struggling with recoil in the future and want a good ethical hunting round that still expands down to low velocities you can always run some round designed for the 300 blackout in it.
The 120 Barnes tac-tx is a great round for the win mag, both at full velocity and extremely reduced. And for larger game you could always run the heavier 190 or 210 nosler accubond long range bullets at reduced loads, while still maintaining great performance on game.