The U.S. hasn't produced plutonium since 1988, instead preferring to purchase it from Russia. We discussed the U.S. government's plans to resume production in 2005, but those plans ended up being shelved.
If NASA is unable to find an additional source, it could limit missions that take spacecraft too far from the Sun. Quoting: "Alan Stern, NASA associate administrator for science, ... said he believed the United States had sufficient plutonium-238 on hand or on order to fuel next year's Mars Science Lab, an outer planets flagship mission targeted for 2017 and a Discovery-class mission slated to fly a couple years earlier to test a more efficient radioisotope power system NASA and the Energy Department have in development. To help ensure there is enough plutonium-238 for those missions, NASA notified scientists in January that its next New Frontiers solicitation, due out in June, will seek only missions that do not require a nuclear power source."