I have used oncidium mixes by both Roberts Floral Supply and RePotMe.com, both with good luck. In a pinch, I'll also use their online mix descriptions to try to come up with my own. Good luck!
I just re-potted my Beallara Peggy Ruth Carpenter. I use fine bark with a bit of moss added in. I soak the bark well first and use a clear pot with good air and drainage holes and just large enough for a few new growths . I like to put some leca (clay) balls in the bottom, but foam chunks or pot shards or other things would work too.