The mass difference between Pu 239 and Pu 240 is so insignificant that it is completely infeasible to use any current production isotope separation techniques (gaseous diffusion, centrifuges, etc.) and Pu 240 reacts to chemicals exactly the same as Pu 239, so you can't cheat it by using a chemical bath to dissolve the stuff you want / don't want (PUREX). There are experimental techniques, but they are so unreliable or expensive that it's cheaper and faster to just build a reactor to make the stuff if you're that serious.
We're talking about national governments here. They don't need to clandestinely take some mixed-isotope garbage from a commercial reactor and recondition it at prohibitive expense and complexity for a weapon. They can just build a short-cycle reactor or one that allows adding and removing U238 slugs while the core is running and tell the UN to fuck off - seems to have worked out just fine for Iran (allegedly), North Korea, Pakistan, and India. The process is really quite technically easy after we figured out how to do it in the 1940s, it's just a matter of spending the money and having the feedstock fuel to begin with, which Japan has shloads of.