If you live near a big city, there will be a industrial dealer specializing in new and used warehouse shelving.
Depending on how big you want it, keep an eye open for shops going out of business.
When a candle store went out of business at the mall, I got all of their store room shelving for $80: It's the heavy-duty knock-together shelves with the 1/2-inch wood shelves. 4-feet long, 18-inches deep, and 7feet tall. I got five of those, and three that were three feet long, all for $80.
I also have a pallet rack in my shop that came from an industrial auction. that one is 10 feet tall and 12 feet long. It replaced the candle shop shelves, which are now in the basement, garage, storage closet in the shop and one in the shop holding power tools and misc.
Brad