Question
How do I re-pot these orchids and the best place to purchase re-potting soil

Answer
The best orchid potting mix consists of medium fir bark particles (1/2-3/4 inch) along with other ingredients such as charcoal, perlite, tree fern, etc.  This mix is what we call "open" meaning it promotes rapid draining of water and good movement of air through the root zone.  You may find bags of it at garden centers such as Lowes, Home Depot, etc..  There are several types of orchid potting mixes, so select the right type.  The more soil-like the potting mix, the less well the plant will do in the long run.  An orchid must be annually repotted in this mix after each flowering because even the best orchid potting mix becomes more soil-like over time which leads to root rot.  Be sure to use a pot that has lots of drainage holes as that also helps drainage and air movement.  Be sure to soak the potting mix for an hour or two prior to use as fir bark, when dry, repels water and soaking it helps to improve water retention.