Last year when it was 90+ degrees outside I bought dry ice at the meat counter at the grocery store. Might try calling a grocery store.
I put 1.5# of it on top of an inch or so of rock salt and then a bunch(~40) of frozen water bottles in my Rtic cooler and they stayed frozen inside the car for 3 days. I didn’t open the cooler at all. Once i did they started melting pretty quick.