The mix they are in is probably Pro Mix ,which is a peat based mix. My farm and garden stores carry it along with other peat based mixes.
If it needs repotting I would use that again, as milts love a constant ,even moisture. Especially if you grow in the home, it is hard to keep from getting horribly pleated leaves. I switched mine over to a peat based mix last year and they have done much better.