Artisan Square didn’t fail me. The lovely Ann (winner of the first Great British Sewing Bee) offered these advice on working with lambswool interlining:

• “You don’t  need to preshrink it – it’s not suitable for use in anything that’s going to be laundered.”
• “It will take heat and steam that you would use on wool, but shouldn’t need much.”
• “It’s a knit – treat it as such.”
• “Don’t stretch it at all when pressing – use a light touch just to smooth it if necessary.”

See the full discussion thread at Artisan Square.

The discussion also confirmed that this one from Bergen Tailors & Cleaners Supply Corp in the US is the real deal. It has thin open criss-cross threads that runs diagonally and thick fluffy threads that run length-wise. It’s stretchy in one direction (cross-wise), but stable in the other direction (length-wise). Someone else identified another US source for it, but this one is a bit more expensive.