What size are you after? If Creality don't make one for their own printer, then that's just another reason to avoid there products.
Flexible build plates are definitely worth it in my opinion. Makes removing very large or really small prints easy and depending on the type of filament you print having the choice of a textured PEI over smooth is important.
Oh it's a resin printer, most of my points above still stand.
I’ve always been mystified as to the appeal of flexi build plates for resin. Surely it’s just more mess, more to clean, and I don’t see the upside. I’ve always just knocked my prints off and put it back, I don’t even clean it unless I’m changing resin or don’t plan to print again for a long time.