So first, what to do with my old one? It could be recycled...maybe? No, unfortunately you can't toss it in the blue bin but you may find a place near you that can recycle it here. And here are a few ideas for reusing it:
Beach Blanket. Or at least something to put under your towel.
Non-Skid Rug Pad. Cut a sized-to-fit piece of your yoga mat and put it under a slippery rug.
Donate to a homeless shelter. This would probably work fantastic in SF.
Gardening knee pad. Cut and bind with twine, or just fold.
This has got me feeling kind of crafty; I think I'm going to cut my old mat into a custom-sized dog bed for my rat dog and make a washable cover for it. It will be like memory foam, sort of. There are also a million more ideas here for reuse.
As for a NEW mat, J. Brown at Recovering Yogi makes a pretty good argument that a PVC mat from a country (like Germany) with reputable manufacturing standards, like the European OEKO-TEX 100 standard, which means the manufacturing process is subject to a higher degree of scrutiny and prevents mats from containing heavy metals and DOP softeners, is a reasonable approach to the balance of usable (sticky not slick), and sustainable (lasts a while). So thanks J. Brown for that balanced approach.
I don't have any experience with natural mats made from jute or the like, but you can certainly look for ones free from vinyl, PVC, and phthalates (an endocrine disruptor linked to cancer, obesity, diabetes). You can google natural yoga mat and find a slew of the eco-friendly mats. I think it would be challenging to make both a yoga mat sticky, which requires certain plasticizers, and durable. As for the one I buy, I think I'll go with this one from Yoga Accessories that is PVC and free of phthalates. It's also thicker than my current mat which is really exciting news for my wrists and knees because, well, they hate me.