Strange. As eevee are actually native to Alola (and abundant on one of the islands, at that), you shouldn’t have any trouble finding eevee-specific formula. Moreover, formula is not typically an item seized by customs, as if processed correctly (that is, not laced with lead-based pigments, as some countries with less scrupulous quality control tend to do), formula contains nothing that would be considered harmful to the Alolan ecosystem or population. You may wish to ask again or look in larger supermarkets. If all else fails, try shopping on Akala Island, where eevee tend to thrive in the wild.
That having been said, using different formulas for baby pokémon tends to be trickier than using generic kibble for adults. Babies require a very precise balance of nutrition that varies greatly by species. However, if you absolutely must, you can give your eevee formula from a different species of pokémon, but (and this is a very important but), it can’t simply be formula for a member from the same egg group. Try to match up the other species with eevee as much as possible. Your target species should be a carnivore, preferably a canine, as these will have the same nutritional requirements (or as close to the same as possible) that your baby eevee will have. Rockruff or vulpix may be your best choices.
Good luck, and congratulations on the new addition to your family!