To be clear, these items rarely go bad, and they are readily available on eBay from a number of different manufacturers. But, the gold gems are the Niles, Speakercraft, and Russound units that have been removed from homes and are basically in 100% perfect shape for around 100 bucks.
Sometimes you can find deals for less.
ie:
http://www.ebay.com/itm/Niles-SSVC-6-Six-Speaker-Selector-with-Volume-Controls-/122728256194?hash=item1c932c92c2:g:1JEAAOSwP4ZZzELd
If you need Phoenix connectors for the 6th output, you can pick them up online or PM me. I have many.
There are other models there for a bit more.
Another option:
http://www.ebay.com/itm/OSD-Audio-6-Speaker-Selector-Volume-Control-/172501882618?hash=item2829e9d6fa:g:ltoAAOSwjDZYijO9
This isn't audiophile quality listening at any point. You are taking a high-level input and sending it to rather inexpensive speakers outside.
This is actually the one I would most likely get:
http://www.ebay.com/itm/Audioplex-OPT-6-Speaker-Selector-6-Zone-With-Volume-Control-Made-in-USA-/122670995994?hash=item1c8fc2da1a:g:e4MAAOSwOTVZph7n
It's inexpensive, appears to be well made, and can handle 200 watts of power into it.
You also have to keep in mind that unless you've wired things to stereo speakers outside, that each channel is 'mono'. If things are currently working, then you really just need 5 volume controls. It's kind of a weird setup because you aren't switching stereo sources between rooms in a home, or adjust individual rooms, but you are dealing with one speaker at a time, each receiving one channel of what is likely a stereo source. Perhaps the input can be set to 'mono' and the output can be set to 5-channel stereo mode, so all speakers get the same sound at the same level, but I would continue to have concerns about the A/V receiver being the power amplifier for multiple speakers which may be driven at high volume, when it isn't what A/V receivers are really designed for.
I love the OSD amplifier w/front volume controls. That's something I haven't seen before. It provides discreet power to the speakers, which is nice, but it sounds like you only need 5 channels of amplification and volume control, not the more traditional 12, which is what a 6 speaker (pair) selector provides.
This search is one to look through:
https://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=speaker selector volume control&_dcat=81741&rt=nc&LH_ItemCondition=4&_trksid=p2045573.m1684