If I understand right, the hard part is purifying the radioactive material. Even if you have access to a uranium mine, there's a lot of work to filter the U-235 from the U-238 or to breed it into plutonium.
It's even harder if you start with other sources. But if you could figure out filtering it, a cubic kilometer of sea water should be enough for a bomb.
US government is very interested in any kind of uranium mining, processing, enrichment or plutonium breading. For example in 1944 US wanted to control world-wide uranium mining.
https://nuclearpowerhistory.com/2025/11/groves-and-uranium/
"The NSG was founded in response to India's first nuclear weapon test in May 1974. It first met in November 1975. The test demonstrated that certain non-weapons specific nuclear technology could be readily turned to weapons development."
https://en.wikipedia.org/wiki/Nuclear_Suppliers_Group
Uranium is not even that rare, it's just that when chemistry fails at separating atoms, you have to use physics, and 3 ~proton~ (EDIT: neutron) masses is very little to work with