Rot-proof spacer under sleepers?
 

I'll soon be putting in a deck on an old (but solid) concrete slab. "Sleepers" will be 2x6 on edge (because I'm extending 18" past the slab). I want to put some non-compressable, non-rotting item under the sleepers and shims to keep the wood up off the slab and out of the water. I've seen plastic Handi-Shims (which aren't really shims as they are constant thickness), but it seems there must be some cheap material, probably made for some other purpose, that I could cut into 3" lengths and put under the sleepers every 2' or so.
I thought about cutting up the aluminum channel from my old storm windows, but it is not solid enough. Could I use 1/2" rebar? the last owner left me piles of it, but I would prefer not-round and not-iron.
The kids don't use the Legos anymore, hmmm.... ;)

I HOPE YOU HAVE A GREAT IDEA - THANKS!

Post a picture of what you have now.
Is this a slab right on grade?
Why not vinyl lattis strips? Lowes has it.

Rot-proof spacer under sleepers?
 

Thanks joecaption - will vinyl lattice hold up under the pressure? I thought I might need a more dense composite of some kind.
Yes, deck is at grade, next to the garage. No door to it, just a step up from the lawn. More a fancy patio than a classic deck.