Here in California, I have yet to find a source of cheap sponges like these. The only thing I've found are really expensive sponges: like, most of a dollar each.
I mean, I know in the global scheme of things it's pretty trivial. I get it.
But generally I miss shops where you can buy good quality things for trivial amounts of money in small quantities. I imagine if I went to Costco I could buy sponges for a couple of pennies each ... but I'd probably have to buy a thousand of the flippin' things!