Get a experience earth work contractor, dig a hole and buy a 20ft or 40ft container

get m&e guy , weld seal the container, install pump and circulation pipe, etc, use salt water if you don't want to use pumps (better to have)

heres the concept from online, google container pool

https://premiershippingcontainers.com.au/as...ing-Pool-12.png

if you don't mind ugly then you can install it above ground, or decorate the walls, or half buried it.