In our garage, I'm trying to mount a flourencent light fixture that is 4 foot long. The problem is, I don't know how to mount it. I'm assuming that I would want to drill the screws into the ceiling joists, however, that's posing to be an issue due to the ceiling joists running horizontal, which is the same way I'm trying to mount the light fixture. Additionally, I can't exactly run it vertically as the only junction box in the garage is between the garage door opener and the attic door. Is there another way to be able to mount this thing?
Are you able in the garage attic? If so you can use the existing junction box and run a wire from it to a new receptacle box where you need it. Or two boxes for another receptacle and another light later. If your garage is unfinished then it will be even easier. If you cannot get to the attic you can use surface mount raceway.
The raceway comes in 5 foot sticks, and has an adhesive on it's back. You can also get clips to attach it to the ceiling and not worry about the adhesive. It's 2 pieces, front & back. Boxes, flat elbows, tee's and many more accompanying part are available.
If your box is round you can use this extension box. They do make these in 1 gang boxes also. It does not have a back to it so you can put it over the box you have and run the wires through the raceway.
To terminate into the box you might use this if you have a 1/2 knockout.
For all the parts available from the company I picked, click here.
The pros on the raceway is it is not too expensive and easy to use. The raceway is made by lots of companies so there will be different sizes available. Different fittings and elbows and terminations should be available and some will interchange with other brands. It should be easy to find at a big DIY box store.
The cons are that it is surface mount and it will be seen.
I'm assuming your garage ceiling is drywall? Why not get some heavy-duty drywall anchors (toggle-type) and just mount it to ceiling?
A 4ft fluorescent light fixture isn't that heavy, the drywall can easily support it, you don't need to anchor into the ceiling joists.