What are you making?
Shallots are in the same family as onions, leeks, and scallions so you can swith any of them in, with a bit of garlic added.
right about now you could also use ramps. ;)
Use a bit of red onion and a touch of garlic and white pepper.
Minced red onion with a bit of garlic is my standby substitution. But then, I always have shallots - I love them!