I don't know but I'd imagine that approaching the manager of any local large supermarket would be worth a try.
The novelty of having someone actually ask for a couple of carts instead of just stealing them would likely be well received.
+1
I used to work nonprofit. Just approach the manager and ask. If he says no, do not assume his answer will be indicative of the next one. When looking for a meeting place, I asked the local public library and was told no. The next guy I asked literally gave me the keys to the building his multimillion dollar business was housed in. I'd ask all of them before I gave up.
Mike
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You can't put a price on cool, but if you could it would be $BLIMP.00