I called my local brew supply shop, and they said they don't have Caramel 60L, but they can combine Crystal 40L and 90L to create a 60L. Are Crystal and Caramel interchangeable, or close enough?
Crysta/caramel are interchangable. The color ratings are not quite though. They each have a distict flavor and color. Combining the two will be close but not exactly the same. What are you brewing?
Personally, I'd just sub in the 40L. 90L gets a few different types of flavors working (dark fruits, etc), whereas I don't perceive as much of a difference between 40L and 60L.