Costco, Trader Joe's, Lowes, Home Depot sell orchids. I haven't been to Lowes, so can't comment on the prices, but Costco and Trader Joe's are pretty inexpensive. What they carry probably depends on where you live somewhat. Same for Home Depot.
I believe Lowes sells plants that are about done blooming at a discount.
Local garden centers and nurseries may also discount orchids that have finished blooming - doesn't hurt to ask
