I also recommend Rainbow Grocery. But look in their bulk foods section, and you can get very small quantities of xanthan gum as well as guar gum (look for them in the big glass jars). Two very popular stabilizers for ice cream / gelato.
Whole Foods also sells xanthan gum from Bob's Mill in 8 oz packages, which should last a lifetime for the home ice cream maker.
Products sold as "ice cream stabilizer" as well as commercial gelato base powder usually contain one and/or the other gums mixed with sugar.
A little bit goes a very long way. Experiment based on what flavor and what ratio of cream and milk you're using. As a general guideline, I'd say about 1/8 tsp is similar to about 1 large egg yolk, so use very sparingly or else you'll end up with a big gummy plasticy blob that will jam up your ice cream maker.
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