Best place to buy Pond fish

Good evening

I'm looking to restock my pond with fish after they disappeared. Not totally sure where they went,  I'm guessing we have a Heron visitor, either that or our cat has got hold of them.

I've read somewhere that herons rarely if ever,  land directly in water so a trip wire type affair will help someway towards preventing one catching my new fish.
I don't like the idea of netting as if the cat is responsible, I don't want him getting into trouble if he falls in and gets tangled up.

My question is where is the best place to buy fish? I'm not after gold or koi, I want something like grass, crucian, mirror or black carp.

I've seen you can buy them online,  is this sensible?

Thanks for any advice

Matt