Rhubarb DOES exist here but you are highly unlikely to find it in a supermarket.  In China, it's usually cultivated for use in TCM not for the dinner table.

It's called 大黄 - dai huang.

Chinese (and other Asian people) are more sensative to bitter tastes than other people, so I think that's the main reason it's not used in food.  Either that, or in TCM it's used as a laxative, so that might also be why people don't eat it here.

Anyway, I've never seen it in supermarkets or even the vegetable wet markets, but ONCE I saw it being sold out on the street by a guy who was also selling other plants,  live snakes (to be used for snake wine), and piles of TCM mystery powder.  But that's all.

Farmers make way more money selling it for TCM over food.  I suppose if there is enough interest, maybe City Shop or something could get some as the stuff is grown all over the place here and especially further south (again, all for TCM).

You should probably just find some seeds and grow your own.