It is a great cartridge and the 336 is the longest run of firearms chambered in it. If you rely on factory ammo it could be an issue. Shelves & web sites been bare for some time, a year or better. I've always cast and handloaded for the cartridge to get a bit more performance out of the compact cartridge. I've owned two Marlin, a 336D & 336SC, still have the "D" which is a 336 based on the Guide Gun series. I also have one of the 3K Rem 600s chambered in the cartridge, wouldn't rid myself of it for anything! I had a Savage 170B pump chambered it and always will regret getting talked out of it. Last year, I bought a CVA Hunter single shot chambered in the cartridge and loved the rifle. A fellow that helps me quite a bit here on the ranch took a shine to it and I sure enough owe him more than that rifle cost for all he does around here so I stuck it in his truck.
Factory ammo is loaded to the SAAMI Spec, 35.0K PSI but you can bump that a bit in the Marlin. I push it a lot harder in the Rem 600.
I started to look through my files for this cartridge and came across this article to introduce my 35 Rem bullet. I had a business that sold bullet molds and in 2006, the 100 anniversary of the 35 Rem, I decided it needed a bullet to honor it's achievement: Ranch Dog’s Bullet for the 100 Year Hunter
I had a 336ER, chambered in 356 Win, but felt there was no real gain in the performance of the two cartridges so let it go. My "D" has killed a whole bunch of hogs...