How to separate wooden shelf support from concrete floor
Here are some different approaches:  


Support it on the wall instead of the floor.
As there are options to place hangers or other support-like parts possible it's an option for clear-floor solution. It isn't my best advice as you will be restricted with the weigth limit (supports). It's just somtimes you want all the floor clear for easier cleaning or (in your case) being safe from rotting.  
Use non-rotting supports.
Why not to use some brick or a concrete block (attached to the floor or not) as a support with tar paper between wood and brick/block. This solution will be a solid one. My favourite as an answer.  
Use replaceable/expendable support.
Why not to use some wooden posts that can be replaced in like a year or so (when you could just make some garage refurbishement)? It's an option when you are not sure what to choose or just can't (not want) to go into brick/concrete stuff. Just remember that these parts (supportive expendables) need to be solidly attached to your posts in some way (you can use metal sockets or connectors).  
Metal parts socket-like with anti-corrosion layer.
Be it a paint or a zinc-layer, you will for sure find anything like that in stores. Some of these are nornally used by deck builders. You can, of course, paint them yourself as well.