Bettas are softwater fish, the residue in their kidneys... It scares me because I have 253ppm hardness in my tap water..
OK. In that case, you are best using RO water. Bottled drinking water will usually contain minerals, much of it is "mineral springs" or similar. Though it may be much less than your tap water, and an improvement. Also watch for iodized salts and such.
Rainwater is usable, if collected in a safe area and collected safely (in a tub/bin, not as runoff from a roof).