Most grocery stores (especially ones with a Mexican products section) ought to carry Tajín. It's also easily found online.
Richard's Rainwater is pretty easy to find in the Austin, TX, area. If you live elsewhere, you'll sadly have to use a different sparkling water to recreate this drink, but at least you got to try some rainwater :)
Get more Mortar & Pestle ingredients
We're selling a Mortar & Pestle Kit on our Make More Store, which includes all of the ingredients for 8 of this cocktail (just add tequila)!
You can also get more of our toasted coconut agave syrup from the Make More Store.
Woodward Extract Co. custom-made their sesame extract just for this box so they aren't selling this particular flavor separately, but there are other sesame extracts for sale both at specialty stores and online. Most should work well for this drink.
Any local store that specializes in mixology/bitters should carry Bittermens xocolatl mole bitters, or you can order online from B&B Supplies.