Farro is a wheat variety, so spelt or wheat berries would be somewhat similar, in addition top the kamut (another relative). I believe the main difference for practical purposes is that farro is available 'semi-pearled' -- with some of the outer layer removed, so the cooking time is reduced.
All the substitutes mentioned would work, with different effect.

I have used kamut before. It is a lot cheaper and very good, but you have to either soak it overnight or use a pressure cooker. It has the same chewiness as farro and should be available in bulk at WF.

I guess it depends how you are using it. If it's a salad I would substitute quinoa, couscous, orzo or Israeli couscous.

I've been told that barley can be substituted for farro though I haven't tried it myself.