Its not that simple. You can't just recover it from nuclear reactor waste because it's mixed in with other isotopes of plutonium, and isn't in that great of quantities to begin with. So first off you have to reprocess nuclear waste to extract the neptunium - which again, itself isn't in very great quantities, it takes a lot of waste, and most places don't want to do waste reprocessing to begin with due to cost and liability issues. You then have to make neptunium targets and expose them to a neutron flux - that is, using neutronicity that could otherwise be used for power generation or other valuable purposes (it takes a lot of neutrons to make a tiny bit of Pu238). Pu238 should be more thought of as a manufactured product than as a byproduct of particular types of nuclear reactors.
There are a few other candidates for use as space power sources that actually are waste products, but they're all significantly worse performers. There are two other alternatives. One is to make a Sterling RTG, which was in development, but funding has been cut off (it's also kind of tricky because you have to ensure that something with moving parts will operate for decades in the harsh environment of space). The other is to make an actual nuclear reactor. This means almost limitless power, but it comes at the expense of not only massive development costs and public opposition, but a large minimum size and massive radiator requirements, as well as the same reliability challenges of sterling generators.
There's no easy solutions. Except, of course, to stop bloody wasting plutonium once we have it.