The Alphabet corporation finally made some decent video recommendations.

After all the lion kingdom's work trying to find something better than $1 flip flop material, this guy came to discover $1 flip flops were the best material.  He obviously didn't run 16 miles in them.  He also mentioned everyone makes sandals from scratch, so you're not going to achieve any fame.  He found door mats to be too soft.  The dollar store here has nothing, but it's a real dollar store.

This guy came to discover the same toe knot solution as the lion kingdom, but extended the double string around the entire sandal instead of just the toes.  Otherwise, he expended a huge amount of effort in leather work that would have been thrown away if he made 1 mistake early on.

Before xeroshoes was making boatloads of cash, the founder made some videos about how to make them.  He dropped some hints about the lace material & soles.  He also has a mane.

The lion kingdom finally checked all the local sources which haven't gone out of business yet.  OSH had the 5/16" fluorescent green paracord used by xeroshoes for $8.  Homeless Despot didn't have the xeroshoes string, but had many other options for a lot less money.  Among sole materials, industrial floor mats were the most promising.  There was also a crazy idea of making sandals out of carpet, so every run could be like moving into a new house & you could feel like $3.5 million, but carpet would absorb water.

Industrial floor mats would be far cheaper than the Vibram materials everyone uses, but there are so many options, it would be more cost effective just to invest in a Vibram material to get an idea of what's proven to work before trying something else.  What they're charging for sole material is comparable to what ordinary rubber sheets cost.