Antiseptic, like most crafting ingredients, is almost never found in its pure form in the world. However, it can be obtained by breaking down junk items which contain antiseptic. If searching for antiseptic, it's advised to search for the items which break down into it.
Antiseptic can be used to make RadAway for the purposes of profit.
As antiseptic junk items are rather uncommon, buying shipments of it may be preferable towards mass crafting.
Most recipes that require antiseptic require blood packs except for RadAway.
With refreshing beverage having low Rads reduction in Survival mode, and the sheer abundance of stimpaks and RadAway, it may be preferable to stockpile materials for the creation of glowing blood packs, as antiseptic and blood packs are often paired in crafting, and to keep Abraxo cleaners and blood sacs for Mentats, Mind Cloud syringe and Skeeto Spit respectively.