Bulgur wheat works as an excellent substitution if you don't have access to farro.
Source: Jamie Oliver
Enlarge Image Credit: David Loftus/jamieoliver.com
FROM THE RECIPE:
Farro is a grain similar to pearl barley, but it's dried in such a way that instead of being fluffy and spongy it becomes chewy and nutty. You may have difficulty getting hold of it, but if you can find it you'll be using it all the time – it has an unusual flavour half-way between rice and couscous (this recipe will also work well with both of these).
Get the Recipe from Jamie Oliver