The plumbing codes prohibit using water resistant sheetrock (green board) in a shower enclosure.
You must use cement board until the walls reach an elevation at least 72" higher than the finished height of the drain opening.
All cut or exposed edges must be sealed with a water resistant sealant.
Cement board may not be used on the ceiling.
Also,you may not install a water proof membrane behind the cement board.
Ref: IRC section 702.4 & 307.2
UPC section 2512 & 807.3
You can find the cement board at all home supply centers.