Distilled water isn't usually recommended since it lacks beneficial nutrients that are present in tap water. The only problem with tap water is that there are harmful chlorines, chloramines, and lots of metals that can hurt bettas and other fish. This is why you use water conditioner (like API for example; incase you can't find any, look for that brand). You'll end up spending a fortune if all you use is the pre-made betta water. It's just more cost effective to use water conditioner for tap water.
Aquarium salt is good to keep on hand in case of emergencies like fin rot. You don't use it all the time, but if you see it and you need to start treating, you'll have it. When I saw that my fish had fin rot, I didn't see it until 11:00 pm at night. By then pet stores are closed, so I couldn't get any AQ (aquarium) salt. It's just something that I really wish I had. I only mention it since fin trot seems to be the most common thing on this forum, aside from constipated fish.
Also, most members on here will eventually tell you to feed for fish pellets whose first three ingredients include whole fish (preferably by name). Omega One Betta pellets and New Life Spectrum are often suggested. I know that OOB is sold at PetSmart. I don't really shop much at PetCo (mainly because there are more PetSmarts in my area, and the PetCos are farther away), but I can't imagine them being much different.