She probably used waxed linen thread. Two places it can be found are Ben Franklin and Amazon. I made macrame bracelets and necklaces from this thread in the 70s, so it must be making a comeback. OR, it could be totally different. Back then it was usually found in hippie shops.

In the 90s I made an Indian style basket necklace with that thread and flat reeds(?). The basket measures about 2" x 1 1/2".

If you google waxed linen thread, you will probably find more places to purchase it.

Could you use waxed dental floss in a pinch ?

I use waxed linen for weaving. I love to order my supplies from both Country Seat: http://catalog.countryseat.com/irishwaxedlinenspools100yards.aspx and BasketPatterns: http://www.basketpatterns.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=bp&Category_Code=waxed

Leather shops have waxed thread for sewing leather

You can also try Fire Mountain Gems website. Prices seem to be reasonable. I get thier emails, but have not yet ordered from them, tho have been told by a relative that orders from them that they are good.

Tami..who uses Nymo thread or Fireline.

Could you use fising line? Would not stretch, but strong.