FB, I don't interface stretch fabrics very much - unless it's to stop the edges curling....
If I'm doing trousers with a Yoga Style waistband, I use elastic. Not so much for a stretch waistband, but to give the waistband stability and a little stretch when pulling on and off. (I hope the waistband police won't come after me)
If I needed to use an interfacing, it would be one I buy locally, but haven't got a name for. It's a fine woven, iron on interfacing, made for use with stretch fabrics, but I use it for everything. If you want a sample, send me a PM.
Jessie