I used to have elastic in my sewing basket with which I mended the cot sheets at work. (Cot sheets have elastic on all four corners to fit around the cot legs. Cots stack by means of openings in the cot to fit the legs. Sheets, wherever sourced and even hand made, covered the openings. When stacked hastily, cot legs ripped through cloth sheets or elastic loops or both. Hence: elastic.)
I gave up mending sheets when tendinitis struck, and much of the elastic being old and scratchy, I threw it all out. So now I have no elastic to make masks with, and doubt the wisdom of going to the dollar store to get any. Or even buying hair elastics which the clever Koreans use to make impromptu masks from paper towels. (Well, towels and filter paper, which they somehow have access to, don't ask me how. The Korean super might have it but I couldn't possibly go in there bare-faced.)
And the question is academic now anyway, because I don't throw things away that I might need. It's merely a question of looking in the kitchen tool drawer, not the paper goods one, and there are my face masks, all precious four of them. So I will hand-sew some double thickness (quadruple? using two pieces of cloth gives four layers) slide ons and wear that for shopping. At least to the Chinese grocery and the Korean super.