The thing is that if you have an established tank, you don't need too much to keep it going and also that some of the items you use have a shelf life.
Any filter media I'm using could be bought in bulk, as it's not going to go bad. Things like most fish foods have a shelf life. Also, I'd not want to keep a massive amount of salt around just due to the space it takes up.
Now when it comes to equipment, if your looking to upgrade something, it might be a good time to do it now if your worried about inflation or lack of availability.
So far I have not done anything special to stock up on supplies.