Ok, uhm, I've recently done a shino costume, first costume actually, and I made my sandals from scratch. Let me explain as bast as I can.
You're going to start with two pairs of cheap foam sandals, you know the ones you find at wal-mart for like $5 a pair. Cut the thong off that goes between your toes on both of them. Put them aside. Now get a fabric that you are comfortable with colorwise and make sure that it is sturdy enough for a sandal and comfortable enough for wearing. I used a sort of cloth with a foam type backing, Couldn't find any at fabric stores, but I did find JUST enough In my garage, so I used it. Next use any technique you prefer to make a comfortable 'boot' shape from the fabric. (I can't quite remember what I did to get mine that way, lol.) Make the ankle/top of the boot about 2 inches longer than you need it. Leave the bottom of the 'boot' open and add about an inch or so at the bottom extra (you will be sandwiching the fabric between the two foam sandal soles) now either leave the back of the 'boot' open or cut it, sew in a zipper of your liking into the back. Now make a cuff at the toe and the top of the ankle. Make sure to not make the ankle cuff too big as you will be folding it down to the desired length. Now that you have your finished boot with the cuffs and zipper, get an industrial grade glue (any kind you want, just make sure it is REALLY strong) and prepare the soles to be glued together (you usually need to apply the glue then let it dry then apply a second coat) sandwhich the bottom of the 'boot' between the sandal soles (this is why you needed to leave the bottom open) and glue them in place. let it dry and you should now have a comfortable fit sandal, slip it on, zip the back and then fold down the ankle cuff.
Hope that helps, there are finished pics of my sandals in my image gallery. Or if you don't mind spending $75 on a pair, order some from www.****.com
they make good ones.