The US does in fact produce plutonium 238 now, not much but production is being scaled up[0]. From what I understand, most of the issues with developing new nuclear power sources stem from funding shortages. NASA canceled their Prometheus nuclear reactor program, which was to provide hundreds of kilowatts of power for a Europa orbiter, because the Constellation program needed more money. Most recently, NASA canceled the Advanced Stirling Radioisotopic Generator because development costs had risen more than expected[1].

Although there might be some encouraging results soon, NASA is supposed to be finishing testing of the new Kilopower nuclear reactor at the Nevada test site soon.[2] Next step after this breadboard test is building an integrated system. If they can get to that point, they will have solved much of NASA's problems with plutonium. A nuclear reactor can use uranium fuel which is much simpler to procure than Pu-238.

[0]https://www.space.com/36217-plutonium-238-nuclear-spacecraft...

[1]https://en.wikipedia.org/wiki/Advanced_Stirling_radioisotope...

[2]https://www.nasa.gov/directorates/spacetech/feature/Powering...