How can I find well-connected nodes? If I am a merchant accepting 0-confirmation transactions I should protect myself against race attacks. Several resources state that I should configure bitcoin daemon to not listen for incoming connections (parameter -nolisten) and only connect to "well-connected" hosts (using multiple "connect=" parameter).

Question: How do I know which hosts are "well-connected" and suitable (long-running, no dial-up...)? Also it seems I can not rely on the mining pools because they probably have a limit on connections which might be already reached.
You could try connecting to various mining pools, fallback nodes or seed nodes. Those should be well connected.

The best option, however, would be to host your own Bitcoin node on a server and set it to connect to a lot of outbound peers. This would also ensure that you would be able to see most transactions quickly and also broadcast the transactions that interest you to more peers.

All in all, it is best not to rely on third-party nodes if you can set up your own well connected node. Also, you should also consider a pre-mined double spend attack that would be undetectable no matter how many nodes you are connected to.