You want either "377x370" or "376x370" size.

TH3D EZFlex system:

https://www.th3dstudio.com/product/ezflex2-flexible-print-system/

Fulament (currently out of stock for E5P size)

https://fulament.com/products/fula-flex-2-0

Wham Bam

https://whambam3d.com/collections/flexible-build-system-for-fdm/products/377-x-370-kit-with-pre-installed-pex-build-surface-1

FYSETC on amazon

https://www.amazon.com/FYSETC-Printing-377x370mm-14-84x14-56inch-Compatible/dp/B09BTV3YBC/ref=sr_1_4?crid=38GEZRQGKSRWA&keywords=ender+5+plus+pei&qid=1651552539&sprefix=ender+5+plus+pei%2Caps%2C92&sr=8-4