I need an alternative to leather
What you're looking for is EVA Craft Foam sheets; these are the typical choice for costumers who want to create leather or metal-look pieces without using leather or metal. They're widely available from most craft stores, typically in the &quot;kid craft&quot; sections under brand names like &quot;Foamies;&quot; lately, more deluxe versions are also available in some stores in the newly-forming &quot;cosplay&quot; sections, often under the &quot;YaYa Han&quot; brand. The sheets are sufficiently thick to replicate leather, but soft and flexible enough to cut with regular household scissors (as opposed to using knives and leather shears). EVA foam won't be as durable as leather, but with care should hold up well, and there are a lot of tutorials available for crafting costume armor pieces using it.
You definitely don't want to do fiberglass; it's very difficult and hazardous to work with, and gets very heavy very quickly.