Same place I get my infinite 2d planes

Those twist ties that come with the new perfume gun you're trying to open.

I got mine from the Lady of Caras Galadhon. She has a nice 7-for-one special she offers, where you and several other people take on a quest to throw something bad into a random volcano. I agreed to throw Lindsey Graham's phone into the Grand Prismatic Spring at Yellowstone, and she gave me some "real elvish rope". Some other group agreed to drop Mitch McConnell's phylactery into Kilauea. We haven't heard from them, yet. You've got to be really smart about what you ask of the Lady of Lorien, too. A few years back, some dolt asked her for a single hair from her head. That group did NOT successfully throw Putin's Ring of Polonium into Eyjafjallajökull.

my local hardware store sells Immovable Objects (well, they try) so i'd assume they have Unbreakable Rope as well