According to Wikipedia, the United States restarted production of Plutonium-238 at circa 1.5 kg a year in 2013 after a c. 25-year hiatus.

Good

the Cassini one used 7kg https://en.wikipedia.org/wiki/GPHS-RTG so I guess it will be some time before new ones are made

They've been exploring smaller designs with better efficiency. Not just because of the shortage, but also because evergy gram counts in a spacecraft...

Well maybe every kilogram, but you got my point. Any savings in mass count, and if you get to cut 1% from the spacecrafts final mass, it can mean a lot in terms of speed and timings for the mission.

Wikipedia link: https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

The thermophotovoltaic cells look lik a very interesting approach, don't know why they haven't been used yet. With them, Cassini could've used 2 RTGs instead of 3, resulting in a mass saving of 57.5 kg (2% lower dry mass from one less RTG) and some extra power. I do take into account that this technology ws probably not developed enough when Cassini launched.