Asked for Epsom salts (magnesium sulfite) at all the pharmacies in Udom Suk today: nope. (In Thai: dii glʉʉa.) Guessing if I ride into town can find, but where?

Indeed you can.  Go to the intersection of Sukhumvit and sois 3/4.  On the left, on the soi 4 side (your left as you're coming from Udom Suk), there's a large drug store a few storefronts down from the gas station.  I buy it there all the time.  Can't remember the price, but it's quite inexpensive.  They sell it in what i believe are half-kilo bags.  The full name is dii gleua farang.  That drugstore in general is quite good; my girlfriend lives in Samut Prakan, but I always stock her up with medicines at the soi 4 drugstore because of their lower prices.

^^I also recommend this pharmacy/chemist, one door down from Bully's pub on Sukh with an open shop front...I regularly stock up there when I'm traveling out to a job for all my blood pressure an diabetes medication...if they don't have it it probably doesn't exist, plenty of generics for the well known medications...