I was in Mollie Stone's in Palo Alto today (3/21) to check out if they had a scent of an essential oil I was looking for. While I was exiting the store, I found that they do sell the flexible ice cube trays. They have a limited selection (hearts, etc.), but there is one I do not have which I plan to buy in the near future.
So if you are looking for some small soap shapes to make they have them in the frozen food section towards the magazine rack.