Hi Dembie,
Not sure if this is what you are looking for but I think it is called plastic elastic. It is inexpensive, commonly used in knit garments at shoulder seams to stabilize them. I've seen it at 'Beverly's' and it's probably at other fabric stores... below is a link to it at Beverly's. Below that is a link describing how to sew it into knits.All the best.

http://www.beverlys.com/catalogsearch/result/?q=clear+elastic

http://sewing.patternreview.com/cgi-bin/review/readreview.pl?readreview=1&ID=486