Orchid mixes are as varied as the people who grow orchids! We offer what we at Tropiflora use ourselves for most varieties, a good general mix great for Cattleya, Oncidium, and most epiphytic species. Note that some find our bromeliad mix useful for terrestrial orchids and even Phalaenopsis.
Our orchid mix is simple:
50% high quality medium Fir Bark
25% medium Charcoal
25% coarse Perlite (Spongerock)
This mix can be augmented with some good quality peat, treefern fiber or other components by the end user to make this mix suitable for other specialty plant culture. Great also for most epiphytic bromeliads as well, especially with some added peat.