I haven't noticed any shortages here. I usually bulk buy online, a month's supply of canned, dry and cat litter at a time. I have a big outdoor shed I can store things in.
Our vet saves up all the free samples of prescription food once they get near the sell-by date and lets me have those. It's usually only a couple of meals worth, as I'm feeding more than 20, but it's a free meal of something new. The cats and I appreciate that.Another idea ... is your vet aware that you feed ferals? If so, maybe mention to them you're struggling to feed your ferals. They might be open to having a donation jar, or even bin, for monetary or cat food donations.