Apparently one source of low-background steel are ships built before 1945, especially military ships as they are large and can be in use for decades. The US navy has scraped a few old ships for this use and there are theories that some WW2 wrecks are being salvaged from the ocean bed for this purpose, but there is no conclusive evidence.
And not just before 1945: lead is used for radiation shielding, but freshly-mined lead is slightly radioactive itself... shipments of lead that were mined 2000 years ago and sitting at the bottom of the Mediterranean since then, much less so: https://www.nature.com/news/2010/100415/full/news.2010.186.h...
