If there is no chlorine or chlorimines in it you should be able to use it from the tap.
usually pure ammonia(no perfumes) can be bought at the grocer/hardware stores.
for that size tank I would recommend a hang on hte back(
HOB) filter. Aquaclear is an excellent filter and easy to maintain.
Most Bettas don't apprecate other fish swimming in their space so when you add more fish with Mary I would suggest some bottom fish such as 3 corys (they do better in groups of three or more) or four or five otocinclus(they also do better in threes or more) or one bristlenose(gets 5-6 inches)
Some people have good luck with more than one female
betta together but most wind up with mass murder and bloody carnage (check out some of the posts in the betta forum) Hope that helps
Carol