To your first query
No. There is no such element (as far as we know) that could have muons instead of electrons. As I read it, you have the misconception that muons are rarer forms of electrons (I might be wrong but I got that impression).
Muons are heavier particles that are related to electrons, except that they're unstable. This lets them pull atoms closer together, thus effectively reducing the repulsive energy needed for the nuclei to fuse. But the problem is that muons aren't exactly stable, which is bad. But they are exceedingly common as part of a pion's decay chain, which is great!
To your second query
Muons arriving on the Earth's surface are created indirectly as decay products of collisions of cosmic rays with particles of the Earth's atmosphere.
About 10,000 muons reach every square meter of the earth's surface a minute; these charged particles form as by-products of cosmic rays colliding with molecules in the upper atmosphere. Traveling at relativistic speeds, muons can penetrate tens of meters into rocks and other matter before attenuating as a result of absorption or deflection by other atoms.
When a cosmic ray proton impacts atomic nuclei in the upper atmosphere, pions are created. These decay within a relatively short distance (meters) into muons (their preferred decay product), and muon neutrinos. The muons from these high-energy cosmic rays generally continue in about the same direction as the original proton, at a velocity near the speed of light. Although their lifetime without relativistic effects would allow a half-survival distance of only about 456 meters ( 2.197 µs × ln(2) × 0.9997 × c ) at most (as seen from Earth) the time dilation effect of special relativity (from the viewpoint of the Earth) allows cosmic ray secondary muons to survive the flight to the Earth's surface, since in the Earth frame the muons have a longer half-life due to their velocity.
-Wikipedia Article
If you are looking at a muon-catalyzed reactor (probably an aneutronic type reactor if this is for a spaceship) or catalyzed direct fusion drive (like the Epstein drive in the expanse (best guess as to what that drive even is)), then you'd want a sort of particle collider.
Muons are unstable subatomic particles which are similar to electrons but 207 times more massive. If a muon replaces one of the electrons in a hydrogen molecule, the nuclei are consequently drawn 196 times closer than in a normal molecule, due to the reduced mass being 196 times the mass of an electron. When the nuclei move closer together, the fusion probability increases, to the point where a significant number of fusion events can happen at room temperature.
Methods for obtaining muons, however, require far more energy than can be produced by the resulting fusion reactions. Muons decay rapidly due to their unstable nature and cannot be usefully stored.
To create useful room-temperature muon-catalyzed fusion, reactors would need a cheap, efficient muon source and/or a way for each individual muon to catalyze many more fusion reactions. Laser-driven muon sources are one possible approach.
-Wikipedia Article
There are three (relatively easy) ways to make muons. Firstly, natural cosmic rays slamming into the nucleons (protons and neutrons) inside the atoms in the outer atmosphere generate plenty of pions. These decay extremely quickly (measured in meters traveled at relativistic speed) into (mostly) muons.
Secondly, proton and antiproton annihilation (antimatter) generates lots of deadly gamma rays and a few pions, which turn into muons. This is where most of the thrust of a beamed-core antimatter engine comes from, as pions and muons are charged.
And lastly, slamming particles (like protons) into other atoms with enough energy will trow off pions, and thus muons, in the general direction that the initial components were, in this case, roughly a beam.
They can then be channeled in the same way as electrons (with magnets) into the reactor or engine plasma where it can catalyze the fusion reactions.
There are two issues you have to deal with to make this work. An efficient source of muons, and getting those muons to jump between atoms fast enough to catalyze more fusion (and give off more energy) than it took to make them.
The second issue can be hand-waved off, but for making muons, you'd need some sort of laser assembly that either uses protons or photons to throw off muons, which you would set up around the reactor or engine and have them throw muons into the reactor plasma.
These would have impactor plates that are expended, possibly some artificial meta-material that is the story's McMuffin. Or the laser could be expendable and highly valuable. What if pirates blow up the engines of massive cargo ships and raid them for the laser assemblies, to sell off on the black market?
What if some extreme form of matter like a special variant of diamonds, or crystal (metallic) hydrogen, or some other exotic material serves as the expendable meta material, maybe mined deep inside gas giants.
Or alternatively, you could use supercritical electron-positron pairs in supercooled suspension (when both particles orbit each other in their quantum ground state, never touching) to generate a sort of gamma-ray laser, which hits your meta material and throws off the muons.
Since we don't know how to build an efficient muon laser to catalyze our fusion reactors, its up to you to pick the handwavium that powers such a device.
