I hate to say it but "It Depends", there are a lot of variables.
Now let me explain. Where I am located I have not seen a difference between locating 44 and 454 ammunition. Having said that there is a big difference in variability. Meaning at my local walmart or gander mountain in 2008 during the run on munitions and firearms I did not see a shortage of 44mag or 454 ammo on the shelf. Though I did notice there were more brands of 44mag on the shelf, Hornady, Winchester, Remington, PMC, Federal, Fiocchi and Magtech. 454 ammo brand were Magtech, Hornady, Winchester. Though 454 was only a few brands there were equal amount of boxes and I mean a lot of both.
Now when I noticed this that very week I had to make a business trip 1 state over, a 6 hour drive or 1hour 30minute flight. I always like to stop through the local shops and none of the chain stores walmart, gander had 44 or 454 on the shelf, but bass pro had both 44 and 454 but only in the Hornady brand.
I think you get the point I have made above. I pay very close attention to these things because you are talking about two of my favorite calibers. I love big bore revolvers, I love them so much (not bragging) but I think I am one of few persons whom have shot a S&W 500 enough to have needed to send it back to S&W to have the cylinder lock detent and timing fixed, and I am one never ever to load pass SAAMI spec, I figured if SAAMI spec doesn't serve my purpose then I move up the next biggest caliber.
I hope this long diatribe helps.