Okay, about strings .....
As anyone playing ERB's is well aware, strings are a bit of a hassle. Can't just stroll into GC/Sam Ash or your local Mom & Pop's Music and pick up a set of strings for a typical ERB, so it takes a little searching at first but there are sources. Some that I know of are:
www.conklinguitars.com go to accessories and you'll find the string sets and single strings
www.octave4plus.com a custom string company, rumor is that they'll soon be making string sets for 4 thru 12 string basses
Invalid Link Removed check the Power Steel sets for some options
Warwick has a .175 F#0 that can be found wherever they deal in the Dark Lord sub-contra bass.
Various ERB builders (BEE basses comes quickly to mind) may be able to source single strings for you. Other string companies like LaBella may have some custom options, too. Generally, since extended range players use a somewhat unorthodox series of tunings for the very low and very high range, as soon as you move out of the comfort zone of BEADGCF-type tunings you start to find options quite limited.
I personally use a LaBella six string flatwound set (760FM-CB-XL .029, .049, .069, .089, .109, .128) with a Conklin .018 plain steel for the F string on my 7. I recently switched my fretless 6 to a set of Conklin Power Steel PS5's with a Conklin .165 Power Steel for the F#0. My fretted 6 has the stock LaBella 760FM-CB-XL flats on it.
Hopefully others will chime in about string sets and how they're using them .....
edit: BTW, a real bargain in a 6 string set can be found at
www.rondomusic.com where they sell a no-name called Indie strings - the flatwound 6 string long scale set is only $24.95 and is very usable ....