I believe it is actually called Flexo Tubing....Not a product specifically designed for the fly tying market, and I have never been able to source any over here, mind you I haven't looked for sometime.
I have however had some samples of very flexible tubing from Richie Johnson of Kustom Flyz in various colours and diameters, as I was looking at tying some flexo crabs myself.
Whilst I haven't yet had the time to get round to trying Richies tubing out, amongst the samples he sent, there was a length of pearly coloured plastic tubing the flexibility and diameter of which IMO fits the bill perfectly
He only offers one size on his website at the moment, (16mm) which would probably be fine for smaller crabs, but he is a really helpful guy, and if you contact him, he may well be able to help you out.
His website materials page is....
- Materials