I know there are simple "y" connectors for connectng a pc to a stereo
receiver, but....my stereo is 120' away (and 1 floor above) my pc.

I have tried the wireless transmitters but haven't been too happy with
the quality.

What type of cable/wire/jacks would I need to get the music on pc to
play through my stereo?

I have tried the following with much success: I connected a 1/8"
stereo plug to each end of 120' of speaker wire. Then, I connect that
to my stereo with a standard "y" adapter w/ rca plugs. Right now,
both of my stereo channels (r and l) only play the right channel from
the computer...what happened to my left channel? Do I need different
plugs/cables??

A Y connection splits one signal into two. You need two separate
signals.

How have you wired the stereo jack at the computer end? There are 3
connections, R, L and ground. You should use twin-core plus screen
cable, leading to two phono (rca) plugs, one R one L with the screen
in common.

It's rather a long cable for an unbalanced line level signal. If
you're happy with the quality, fine. If not, things will have to get
a little more complicated :-)