Partial answer by a non-expert.
The thorium as 232Th itself will be a very, very small part of the mass of a safe, reliable thorium reactor. I assume that it will pelletized and pretty safe if you don't ingest/inhale it. It's not actually radioactive, and used to be given to people to eat as a contrast agent for X-rays, so it really could be sent from Earth as a few kg carry-on along with one of the passengers (again, as long as they don't eat it).
Thorium 232 is fertile, meaning in the reactor you'll convert it to Uranium 233 with a thermal neutron source, and whatever that is is a different challenge and you might not want that in a carry-on bit of luggage.
According to World-nuclear.org's Thorium(Updated November 2020) those
fissile driver options are U-233, U-235 or Pu-239
so back to square one with the danger of launching. Mining in space is a way around that. You could launch everything else as non-radioactive payload but find your fissile neutron source (required to use thorium) in space somewhere.
Now you've still got to build a big nuclear reactor, and it's going to have a lot of exotic high temperature materials in it that are difficult to process as raw materials and then manufacture in to components.
Don't worry about the Thorium! It's the least of your worries. Worry about the U-233, U-235 or Pu-239 neutron sources.
"But I asked where!"
Thorium 232 is stable and primordial, and you'll likely find it mixed with Uranium.
The 2010 Geophysical Research Letter Uranium on the Moon: Global distribution and U/Th ratio presents an analysis of gamma-ray maps of the Moon from JAXA's Kaguya gamma ray spectrometer. They don't measure Thorium directly, but instead they pick up a 2614.5 keV line from the beta decay of Thalium 208 in the decay chain of Thorium 232.
Yes I said that it is stable, but it alpha-decays with a half-life longer than the age of the universe.
They detected Uranium 238 from a 1764.5 keV gamma ray line emitted as a result of β decay of Bismuth 214 which is in its decay chain.
Figure 3. Distribution map of U on the lunar surface measured by Kaguya GRS. The abundances were determined by peak-fitting analysis of 238U 1764.5 keV peaks. Labels on the map indicate the following lunar topographies: A, the Apennine Bench; C, Copernicus; I, Mare Imbrium; J, Montes Jura; S, South Pole-Aitken Terrane; and T, Mare Tranquillitatis. Dotted squares labeled as E and W indicate specific highland regions defined as East and West Highlands, respectively (see text). The shaded relief in Figure 3 and 4 was drawn using topographic data by Kaguya Laser Altimeter (Araki et al., 2009)
Figure 4. Distribution map of Th on the lunar surface measured by Kaguya GRS. The abundances were determined by peak-fitting analysis of 232Th 2614.5 keV peaks. Labels on the map are the same as those in Figure 3 (see also text).
What your reactor might look like, from Use of Thorium in the Generation IV Molten Salt Reactors and Perspectives for Brazil
Figure 1. Scheme of Molten Salt Reactor (MSR) (US DOE, 2002).
But you might instead see if a variant of the Kilopower reactor built for space applications can be rigged up to work with thorium 232 somehow.
In your reactor your neutron source first converts thorium 232 to uranium 233, then your neutron source helps to induce fission in your real fuel, uranium 233.
Figure 3. Nuclear reactions involved in the transmutation of 232Th to 233U
update: Here's a map of Thorium in higher resolution from the Lunar Prospector spacecraft that orbited a few years later. Looks delicious!
Map of the Moon (left: nearside; right: farside), generated from gamma-spectrometric data collected by the Lunar Prospector vessel. The map shows the global distribution of the element Thorium in surface rocks, with high Thorium concentrations indicating the occurence of the so-called KREEP rocks, which have high contents in Potassium, Rare Earth Elements and Phosphorus. The heterogeneous distribution of KREEP rocks at the moon’s surface implies fundamentally different geological histories of the individual lunar regions (terranes). For further information see Jolliff et al. (2000)1
1Bradley Jolliff, Jeffrey Gillis, Larry Haskin, Randy Korotev, and Mark Wieczorek (2000): Major lunar crustal terranes: Surface expressions and crust-mantle origins. Journal of Geophysical Research. 105(E2): 4197–4216, doi:10.1029/1999JE001103.
The paper is long and excellent and goes into great detail, it may be particularly helpful for someone wanting to figure out where to start looking first to find Thorium and Uranium necessary to power their reactor after a heck of a lot of refinement.