Does anybody know of a good (low cost) online source for Dupioni Silk or Poly Dupioni that has a wide selection of colors? I've been searching but can't find much.

Not sure where you are, but JoAnn's Fabrics has polyester shantung which is "faux Dupiono"...it is on line and there are always coupons that can be used if...and sometimes free shipping coupons as well...the link is below...

You probably are from the USA? I could give you recommendations in Germany only :-) Good luck for you
and Welcome to CUTE Family -
Maria

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Have you tried fabric.com? I've seen some really gorgeous colors there, and sometimes it's even on sale. Best of luck, hugs, Marji