Can't find leeks at my local grocery. Is there a substitute?

Spring onions, pearl onions, shallots, green onions or if you don't mind a more intense onion flavor, a regular onion will do (unless its a dish centered on the leek).

If you can find sprin onions, those should work. What is the recipe?

It depends on the recipe but leeks are from the onion family so you could substitute with scallions or shallots