I am sooooo sorry for your loss, losing pets is always a heart breaker =( I don't know if it's possible to cremate a fish, but one thing you could do is bury her in soil in a pot and plant flowers over her.
My Current bettas:
Sullivan- Orange rose tail halfmoon male
Daisy- Mustard Gas (black and yellow) veil tail female
Milton- Teal butterfly halfmoon male (mom's betta)
DFTBA to nerdfighters working to decrease world suck!