Add one. Cut a hole In the center bottom and add a bulkhead fitting.
The pools do have side drain plugs but I never use them.
The enitire bottom of the pool is resting on decking except for a hole in the decking below the bulkhead fitting and where the drain pipe fits in.
I've also seen this done on flat ground for a quarantine koi pond where a trench was dug for the drain pipe.
The fiberglass tanks are nice but the cost is over 5 X the cost of a larger Intex pool, not counting the truck shipping.
Keep in mind the Intex pools I am referring to are not the inflatable ones. And indoors they last quite a while. I know shrimp producers that are using the same ones after 7 years.
Here is one I used as a sorting tank when I seined one of the ponds this spring.
