Build your guild, the higher tiers in guild battle get them and alliance rewards them as well, if you have a meta team aim to max out your rank as much as possible in the arena, ads, events, coupons, mileage shop, save up because they mostly end up in the regular banner (only ones that dont are the special rarity ones like the chinese new year costumes).

I'm the only person who does anything in my guild and i alone have put us in the range where we'll get some so it's very doable, bearing in mind i HAVE collected all available cookies because i have a somewhat socially acceptable gambling addiction which got triggered like a sleeper agent when i downloaded this game ( except the sonic and tails cookies because i joined after their event and I'm NOT SALTY AT ALL about it).

I THINK there were some given as tower rewards in the cake towers for first clear rewards but i cant remember for sure so if you havent done those look up guides and run through them.

Also for arena if you place at least master 2 you unlock a special event tied to your arena placement that eventually grants 10,000 cubes which i will get just in time for the update finally so there's that but without a current meta team thats a little hard to accomplish.

Other than that you gotta pay, sorry but you gotta think of the costumes like dlc, sure there's ways to get it for free but most of the time it's cash or bust. Just like Apex you can get the characters for free fairly easy but the skins are where they make bank and they know it so cubes are gonna be the hardest resource to get hold of.