My local grocery stores (all two of them) never seem to have shallots on hand. Oh, occasionally, I might see them on the shelf, but that's always when I don't have a need for them. Isn't that always the way?
Luckily, red onions make a suitable substitute for shallots, and I can always find those. The substitution?
Use half (1/2) of a small red onion for every large shallot.