For some reason, the adapters are hard to find. You'd think it should be easy since you're probably not the only one who'd like to share the pedals between multiple bikes.
Here's one in Japan:
CYCLETECH-IKD : wellgo QRD Quick Release Pedal Adapter
Here's the other version - but apparently compatible - on Amazon Japan:
https://www.amazon.co.jp/wellgo-ウェルゴ...dp/B00HJ3CUK4/