Depending on where you live I would try to locate a salvage yard.
They should have everything form old antique hardware to street lights. Every large city has one. As a contractor I have saved many a comercial owner money buying comercial grade floresent lighting. There are many types. The cheap fixtures at home centers don't last. They might carry a good quality lite but I buy all my new ones at the whole sale house.
I use double 8' floresent hanging fixtures in my both of my shops. It lights up like a runway, day or night. I have them every 4 feet and I can turn them on by every other one or by the row.
I am not sure of the going rate for used fixtures. The price will vary on the number you buy, brand and where you buy them.
Just do not buy the cheap 4 ft home ones. They are pure junk and do not last.
I would run a seperate circut braker for the lights and put in a seperate switch. Do not use the garage switch or light to run them. Ceiling lights should be on thier own circut but many times they are run on the wall out lets for a garage. If you blow the outlet you loose your ligths and this is a pain at night.
I would run all the wire in conduit. Romex has not been tested in conduit for heat so it needs to be single wire if you use conduit. With that said.....
If you use romex make sure you strip the ends way back so you can't see the casing and only the individual wires. If it is ever inspected they can not tell. I did not tell you to do this but it is ok. There is no real reason you can't use it but it has not been tested for heat generation and fire rating. I use Romex all the time for myself but never on a job. It has been cheaper in the past but wire has doubled in price over the past 8 years.
Six eight foot double fixtures should give you enough light to see the dark side of things. LOL I have 8 hanging in my garage. If you look up you are blinded by the light. LOL
Hope this helps. I know the comercial grade cost a lot more but in the long run they are well worth the money. Even if you install them using conduit you can take it all down and take it with you.