The reason that USA buys rocket motors from Russia is that the technology to do so does not exist in the USA. That might change at some point but right now this is a lost art for the United States.
The design of the motors was licensed to the US many years ago but the metallurgical skills necessary to construct them is not available in the US.
The plutonium deal had great advantages to the USA, apart from representing a decrease in the number of nuclear weapons in the Russian armoury. The plutonium represented 'free' fuel for US nuclear reactors. The problem came when the US chose to bilk the Russian people. The deal was that plutonium would be reprocessed so that it could not be used for nuclear weapons. The United States reneged on that deal by changing the reprocessing procedures such that the plutonium could be turned back into weapons again. The process was justified on the basis of cost. At this time, in effect, Russia has been sending weapons grade material to the United States and the US had been receiving the stuff, in essence Russia has been supplying, for free, the raw materials for weapons to be used against its people. Not a situation that could be allowed to continue.