Is ribbed cable housing still available?
 

...white cable housing with ribs running lengthwise - as on old Raleighs of the 1960s and 70s?

To my knowledge, no.

This is about fashion, and ribbed housing went the way of spats, and I doubt we'll see it come back. You might still find some NOS somewhere, but it'll be very old and, if white, likely to be yellowed.

It may be available on ebay, NOS at a price. I have a couple of feet of the stuff that I couldn't toss out, just because. I think that old bikes are the only source now.

Yeah, your best bets are to take it from an old bike or to find something like this on eBay:

http://www.ebay.com/itm/Sturmey-Arch...h/232240448676

some that you could consider

from Italy
https://c2.staticflickr.com/6/5049/5...cdfbd750_z.jpg

from Japan
https://image.jimcdn.com/app/cms/ima...1415/image.jpg

https://image.jimcdn.com/app/cms/ima...1376/image.jpg

or this from Britain
http://www.universalcyclecentre.co.u...kes/bc001L.gif

or this
http://www.universalcyclecentre.co.u...kes/bc002L.gif
but no green

Not available new. I've always called it fluted cable casing. I keep every bit I find. Nice yellowed fluted casings look great on 60s and 70s Raleigh Sports. It's even harder to find black fluted cables for roadster shifters.