I'll admit I've never used these in particular, but as others have mentioned, some options include:
- Locust bean gum,
- Psyllium husk,
- Cellulose gum(carboxymethyl cellulose),
- Flaxseed meal,
- Chia seed
All of these are thickeners and emulsifiers and can help make a gluten-free dough more elastic(what gluten usually does). If you wanted to, you could make psyllium husk yourself by finding plantains and collecting their seeds. If you live somewhere carob grows, you can also make locust bean gum from its seeds.
Aside from xanthan gum, the most successful gluten substitute I've ever used is glucomannan, or konjac flour. If you've ever heard of shirataki noodles(brand names like "Miracle Noodles" or "Skinny Noodles"), this is what they are made of. Glucomannan is a thickener that easily gels water and aqueous solutions and adds elasticity to doughs and batters. It consists entirely of soluble fiber and, in my experience, works better than xanthan gum as a gluten substitute. Like xanthan gum, though, when using it, be careful not to use too much, as it can keep your baked goods from cooking all the way through. The most I could imagine you'd need is around .5 tsp glucomannan per cup gluten-free flour, but it depends what you're making. I'd recommend substituting it on a 1:1 ratio for xanthan gum if a recipe calls for it and adjusting from there if need be.
Another thing about glucomannan is that, when using it, you may have to decrease baking temperature and increase baking time to better cook whatever you're making fully and evenly. If a recipe calls for 30 minutes at 350 F, you may want to try 40-50 minutes at 275-300 F, testing for done-ness, and leaving it in for longer if need be, then adjust the procedure from your experience for next time.
Glucomannan can be purchased in bags online(possibly under names "konjac flour" or "konjac powder"), or can be found in capsules online or in health stores. If you get capsules just empty them out until you have the amount you need. Assuming you have pure glucomannan capsules(the package will say "pure" or "100%"), the stuff inside is the exact same powder as what you need. Good luck!