I must admit i've never seen collard greens in supermarkets as far as I know and I work in one!!!
Collard shares the same botanical name as Kale and is fairly similar so I imagine Kale would be a decent substitute? Not as good but if its all you can find then use that.
I would suggest going to a garden centre and asking them as they may have or be able to source a plant or two for you.
Iceberg is a no-no as is spinach. Collards are part of the cabbage family and I imagine cabbage and broccoli etc would be ok too!
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