Hi,
Looked up and down, slim pickens. Most are powerline based which according to reviews means serious range issues, static, reliability problems. Newer ones need three-prong outlets which is often not possible where you have these little two-wire extensions (what were they thinking?). The Chamberlain NLS2 is 900MHz based which is very good, but according to reviews needs batteries or a really clunky wall wart, and quality was occasionally reported as not too great despite the high price.
(street price is around $100)
What's needed is simple: A box with a "Talk" button and a volume control potmeter. Ideally for 115V two-prong plug-in but where the communication happens on 900MHz or some other RF band that is as low in frequency as possible. Any experience?