Most hardware stores have some sort of complete heavy-duty shelving
unit. When I moved into a small 5x5 storage rental, I bought several
of the sort that are steel, include all hardware and pressboard
shelves. They're roughly 2' by 4' and 3' high, and can be stacked
(which I did).
They were well worth whatever they cost - maybe $40 each?
When I've needed custom heavy duty shelving in a funny size or
spacing, I build my own out of 2x4 and plywood. You can make very
stable shelves by using 2x4s as uprights and notching them with
a circular saw to support cross-pieces. Hmm. That's not going
to be obvious...
Take a 2x4 and lay it on the ground, thinner edge down. Put another one
parallel to it, maybe a foot away. Not take another and lay it across
them at right angles, wider edge down.
Mark the intersection - you'll have two parallel lines on each of the
bottom 2x4s. These are the uprights. Remove the cross piece.
Set your circular saw to 2x4 depth. Cut along the lines you marked.
Smack the section with a hammer, hard - now you have a 2x4 shaped notch.
Drop the cross-piece back into place and secure with a 4" Grabbers.
Repeat for the back set of uprights. Cut two 2x4s to the desired depth
and put everything together to form a square shelf support. Make the
shelf out of 1/2" ply and screw it down.
This is very stable.
I've made a number of 'custom' setups that fit very tightly into a closet
or other small space. Measure, cut and assemble out in the open, mark
the joints and disassemble. Then you transport it and assemble in place.
A lot more work than buying a $40 kit!