Hemp is the best alternative for the main part of the shoe where possible as it doesn't need much water, virtually any pesticides or gmo's. Generally speaking you're mainly looking at rubber for the soles, although it is actually possible to get hemp rubber/ plastic, too, although I'm not sure what kind of processing agents that takes and is much rarer to find.
Something else to consider - buying from a thrift store so you're not encouraging new manufacturing at all.
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