Walmart, or other pet stores such as PetCo/Petsmart, and LFS should carry the salt and conditioner (don't use salt unless needed).
As far as the fish itself- most males will have the longer fins, but there are plakats which both sexes have short fins.
Both males and females are very personable, and have great personalities and are individual in them. The males may tend to be more colorful, but the girls are so cute too lol. So basically, it's whichever one you fall for!
If you go to PetCo, don't get the "baby bettas" they are selling, as unless you are experienced, it's not wise- they require a little extra care (grr on them for selling them so young).
Also, if you can, if you get a male, a cave of sorts would be loved by him- most males love to sleep and lay in caves.. I know mine does, I took them out once to change out the scenery and they didn't like it one bit lol.
A hint: Keep the cup they come in- they come in handy when you do large water changes or have to QT them for being ill.
As mentioned before, want to see pics when you get it all set up!