They don't make it easy to find the Linux products, that's for sure! I did find some here https://www.lenovo.com/us/en/think-workstatio...series-towers/ThinkStation-P520/p/33TS3TPP520 but the price tag for that option is HUGE! It's the "developer AI" version of the standard one.
I personally would NEVER buy one of those as a way of getting Linux!! For those looking for an affordable custom-built workstation with Linux pre-installed and with no other OS, I'd recommend ThinkPenguin instead.