You can buy cubits from players, buy them in the store. Sell items on ah for cubits. Hallow said already /cow and being online to enter the hourly giveaway.
Also /top pays out cubits every month, got to be top 5 for that. P1 gets 50, p2 gets 40, p3 gets 30, p4 gets 25 and p2 gets 15.
Check discord/forums too, sometimes they giveaway keys or cubits through those platforms.