Looking for a quote and source from Brigham Young stating he
believed the moon and sun were inhabited

  I can answer your question, however.  Here's Brigham Young's statement:

Oliver B. Huntington recorded in his diary:
"The inhabitants of the moon are more of a uniform size than the
inhabitants of the earth, being six feet in height. They dress very much
like the Quaker style and are quite general in style or fashion of
dress. They live to be very old; coming generally near a thousand years.
This is the description of them as given by Joseph [Smith] the Seer, and
he could see whatever he asked the Father in the name of Jesus to see" -
Journal of Oliver B. Huntington, Vol 2, p 166, emphasis added

from Brigham Young:
"I will tell you who the real fanatics are: they are they who adopt
false principles and ideas as facts, and try to establish a
superstructure upon, a false foundation. They are the fanatics; and
however ardent and zealous they may be, they may reason or argue on
false premises till doomsday, and the result will be false. If our
religion is of this character we want to know it; we would like to find
a philosopher who can prove it to us. We are called ignorant; so we are:
but what of it? Are not all ignorant? I rather think so. Who can tell us
of the inhabitants of this little planet that shines of an evening,
called the moon? When we view its face we may see what is termed "the
man in the moon," and what some philosophers declare are the shadows of
mountains. But these sayings are very vague, and amount to nothing; and
when you inquire about the inhabitants of that sphere you find that the
most learned are as ignorant in regard to them as the most ignorant of
their fellows. So it is with regard to the inhabitants of the sun. Do
you think it is inhabited? I rather think it is. Do you think there is
any life there? No question of it; it was not made in vain. It was made
to give light to those who dwell upon it, and to other planets; and so
will this earth when it is celestialized. Every planet in its first
rude, organic state receives not the glory of God upon it, but is
opaque; but when celestialized, every planet that God brings into
existence is a body of light, but not till then. Christ is the light of
this planet." - Journal of Discourses Vol. 13, p.271.