I bought a cable to control my 706 from K1NU. His web page is at
www.k1nu.com. Unfortunately he is on vacation until mid-August.
You just need to be sure hamlib supports turning RTS on as that is what powers the interface.
You can get a similar cable from RT Systems at
http://www.rtsars.com. The cable you need is the CT-29A. I have one for my R10 and it works on the 706 as well.
73,
Alan N5NA