i always keep: plain epsom salt, aquarium salt, and IAL
epsom salt treats swim bladder disorder, mild popeye, and helps constipation and bloating and such
aquarium salt helps infection of most external diseases, fin rot, any open wounds etc.
Indian Almond Leaf is a natural plant that when added to the water, releases tannins that have antibacterial/antifungal properties as well as stress relieving properties - you can get these from amy lee on ebay
I also keep Maracyn, its a common antibiotic that treats a wide variety of diseases. But i have 80+ bettas, im assuming you only have one. So the chance of needing to use Maracyn is pretty low. If you have the things listed above, then it wont be too much of a big deal to treat your fish with those, until you can get to the store for antibiotics should you need them