First, though maybe you realize this and there’s some reason it won’t work for you, it’s pretty easy to make, no?
City Super in Sogo probably has the best selection of baking items. Pricey though.
There are a lot of small “DIY” bake stores, the list here is a bit old but should still be usable:
I can’t recall seeing tinned icing in any of these places though. Maybe the big baking stores will sell premade in the fridge?