As stated by another poster, I vote for Superstrut / Unistrut and make your own. I looked at pallet racking too but even used it's WAY expensive and you only have certain choices of shelf depth, and total height based on what is available used when you are looking.... I built two shelves out of unistrut and I love them and they do hold lots of weight. I use 3/4" OSB subflooring to prevent any sagging, but my shelves are only 2' deep. In the below youtube video, scroll to about 6:00 to see it assembled. The video takes you through the build, step by step. One of my shelves, I made the uprights 10' tall, and the shelves 10' long with a center upright at 5'. To hold more weight, I'd just use two evenly spaced uprights. I like the unistrut because I can make the shelves to any size I want. Although not as strong as heavy duty pallet racking, certainly stronger than any "shelving unit" you'd buy at any big box store. Pallet racking seems affordable, then you figure out how many uprights and cross beams you need for the number of shelves, add it all up and used pallet racking is pricey.
To build a unistrut shelf you'll need a 3/4" wrench, a tape measure, and a saw to cut the strut to length, as well as a wood saw to cut your OSB or plywood. I used a 4x6 metal cutting bandsaw but a portaband or abrasive chop saw could be used too.
Strut is cheapest at electrical supply stores, and the strut nuts are cheapest on ebay. The bolts you can buy by the pound at TSC, no need to buy overpriced bolts at the electrical store. Brackets can be found at mcmaster, grainger, your electrical supply store or ebay. Sometimes you can find used strut for $1/ft instead of the usual $20 per 10' stick.
Brian