I need them for my spacecraft, but on this primitive planet no shop
offers nuclear batteries.
This is bad, earth is the LAST place in universe.
Since no-one has given you a sensible reply, here's one.
Modular nuclear reactor cores are used on current naval vessels, ships
and submarines, around the world. These have a fixed useful life
(often about 10 years) and for minimal down time are intended to be
replaceable quickly. You don't want your aircraft carrier in the
middle of a 12-month refit for its powerpack when the sh*t hits the
fan.
Check out the contractors that supply the various military powers with
these - especially ex-soviet block ones. I'm sure they could help you
out.
If you need something even smaller, try NASA and its contractors.
Nuclear power units are *not*, repeat *not*, that's *never*, used in
space, of course, that would be politically incorrect, but you never
know what you might find on NASA's surplus sales.
Only thing is, you didn't specify a budget. For a cheaper solution, I
have some nuclear materials around from my geology days. With a
reversed peltier chip I could make you a power unit with a few hundred
year's useful life at a few nanowatts output. Cost will be in the
region of $25,000 - a lot cheaper than the above options.
If you want automated control, I would recommend a PIC 16F84 based
solution running from a car battery for longevity.
Let me know if you are interested in pursuing this - the summer
holidays are approaching and the nurses let me out for some fresh air
in the sun - I'll have time to plan then.
;-)