I am always on the lookout for suitable and accessible fusible interfacings (underlining) for stretchy necktie fabric.  I found and have tested a Pellon product that I found last week - EK 130 Easy Knit for a light-weight/sheer fabrics or as an underlining.  This product is about 20" wide and comes on a 15 yard bolt.  Cut lengthwise, it is just a tiny bit heavier than my favorite (HTC's Touch o' Gold 2), but still produces a drapey and easy to handle fabric.  Because it is a knit, you will still have crosswise stretch.  And I found it in a chain store - Jo-Ann Fabrics.