You can find both non-return valves on ebay now, the umbrella valve inside the liner and the dome-shaped valve in the pump:
https://www.ebay.com/itm/Non-Return-Umbrella-Valve-Check-Valve-Seal-for-Bestway-Coleman-SaluSpa-Lay-Z-Spa/254813890356
https://www.ebay.com/itm/2x-Air-Blower-Non-Return-Check-Valve-Seals-for-Bestway-Coleman-SaluSpa-Lay-Z-Spa/254798466900