I don't know if there is such a thing. None of the grocery stores where I live in the US have it. I did find a source where I can buy canned rhubarb. It is a catalog. Go to vermontcountrystore.com and see if you can find it there other wise ask for one of their catalogs. It isn't in every catalog, but in most of them. It is the only solution I have found for rhubarb, because I don't know anybody that grows rhubarb.
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I have never seen frozen rhubarb. However go to vermontcountrystore.com every now and then they have canned rhubarb. I think it is rather expensive, but to me it is worth it because anybody that I know doesn't seem to grow it and living in an apartment complex I can't raise it. The canned does taste good right out of the can.