Arrrgggg. I know that these exist, but I haven't found what I'm looking for on the internet. Sorry. Hopefully someone else will pop in with a link to something.
If we can't help you, you could always ask a local A/V shop what they use. Every one that I've been to has had the ability to select which amplifier is powering a given set of speakers (in addition to picking which set of speakers is being powered by a given amplifier).