I've seen a few players who had the fishcake, and though I'm aware it is just a reskin of another food item, I still would like to have it. I checked the Mann Co. Store, but it's not available for purchase. Is my only option to trade for it, or are there other options?
The Fabricate Class Weapon blueprint can no longer create this item, but it is still craftable by using three Moustachium Bars and a Fish, which are promotional items earned in SpaceChem. SpaceChem can also give the items required to craft the SpaceChem Pin, another TF2 item.
Source: TF2 wiki
I think if you have crafted it before it was removed then you would still be able to have this item.
If a player crafts an untradable Fishcake, clicking on the Fishcake item in their >backpack and pressing the Restore button will make it tradable.
hope this helps.