They're pretty easy to make; back when I cosplayed Naruto, i made my ninja sandals this way; they're not quite perfect compared to heavy-duty sewn ones, but they work well and are quite cheap (here's the ones for my Sai cosplay, though I never took any of the sandals themselves- http://www.cosplay.com/photo/1570904/
and though the colours didn't match right, my first pair for my first cosplay... http://www.cosplay.com/photo/1519583/
1- Get a pair of fairly cheap, foam-soled flip flops that match your foot size and the colour you're going for. I bought laguna brand ones from Walmart, but I dunno if they come in men's sizes.
2- Get a pair of decently-stretchy socks in said colour; snip off the toe and heel, then slice open the bottom of the foot to essentially make it only held on by the ankle.
3- Stretch those open sides over each side of the flip-flop, and with a strong needle (and a thimble! You'll need one) stitch into place. I add some superglue as well to hold it on better. You're essentially sewing on a diagonal angle through the sole.
4- Take some good-quality felt, cut into bands (straight across for the toes, on a u-shape for the heel) and stitch in place over the edges of the socks and to the soles, to both hold it to the sandal better and hide the rough edges. The felt does stretch over time, but it's easy to tuck and add a few stitches to tighten again or replace altogether. You could probably use another non-fraying fabric, but I've always used felt so I can't offer suggestions there. An added bonus of it being flip-flops, is they're far easier to walk in than completely open ones. Hope that helps!