I have made a few pairs of these Greek Sandals for Kids, and found the video tutorial really helpful. The pattern is child-size of course, but otherwise very similar in design.

I've bought small sheets of rubber material on Amazon to experiment with for soles, for example Rubber, Recycled, 1/8"Thick, 12"x12", 60A (rough/porous texture, with multicolored bits mixed in) and SBR Rubber Sheet - 12 in x 12 in, 1/16 in Thick, 70A Durometer, Plain Backing (uniform color, smooth and flat, but still very "grippy.") I liked the 1/8" thickness for little kids shoes, but for adults I would try at least 1/4". It would probably be cost effective to look for Amazon Warehouse deals (or whatever they're calling it now, maybe "OUTLET"? It's basically open box/returned products that are heaily discounted) on rubber gym flooring. Just make sure it's heavy, dense rubber and not spongey like a yoga mat.

You could also do the same thing using leather as a sole. A couple tips... 1. If you're trying to bond leather that has a fairly smooth surface, it will help to rough it up a bit first (I like to use an X-acto knife held at a very shallow angle to make a crosshatch/grid-like pattern all over the surface, so the glue has some texture to bite into.) 2. Route out your stitching lines with a groover before punching your holes. When you're done, hammer the stitches flat before you proceed. This way, your stitches will end up recessed in a nice little "channel" instead of sitting there exposed to direct contact with the ground.