I’m unfamiliar with how Skyblock works, so I’m hoping someone who is knowledgeable about this corrects me, haha.
But I’d suggest checking /shop for rabbit spawners or even just rabbit mob drops, as for salmon, see if there’s any categories that have it. I’d just like to ask how you can’t catch salmon? Surely you’d be able to. If not, I suppose you could ask if anyone is selling salmon.
On skyblock we have a particular fish system. When you catch a fish, it's a normal fish that has both a name, rarity of the fish and the size. Although there are other things that can spawn from fishing, I'm pretty sure there is little to no chance of getting salmon but dont quote me on that.
Also with rabbits, mvp+ can spawn them in or ask someone who has obtained rabbit spawners to kill some. There isnt rabbit spawners in /shop but rather in the Easter chest... which goes soon...
Oh well I'm late to this...
You can cook any fish for the challenge. - But you knew that already.^^
Rabbits however you can find at /warp stuff if you go 1x down where all the other friendly mobs are.
I hope that still helps. <3