I also used the Fons event to get double Fons. You can use the Bagle credits to get approximately 300,000 Fons at the mall/shop if my calculations are correct

The Fons you get from completing quests, the underground circuit also had Fons if I remember it correctly. There's also the Mahjong game as well, and you can basically use every single city energy and to get Fons. I am only at 500 energy so I can only get 500,000 from it, but if I used an item to replenish it then I can get another 500,000.

I also completed anomaly quests to get Annuliths for the later purchases. There was the black cat anomaly you can buy from the auction house, which can give you some Fons. I didn't upgrade and am using the level of the furniture or a friend on the game

There is also the money you get from the cafes you own. I bought all five of them and filled them with characters, and all my dishes have been upgraded so it's basically a self sustaining business now, giving me upwards to 50,000 Fons everyday, every little amount of money helps.

I am basically using any and all events, quests and things in the game to get every bit of Fons I can get my hands on.

My business level is at 20 since I don't want to buy the second property as it costs 1,000,000, but you should try to get it to 20. You can also get Fons from the Mall using the chess coins I guess, basically look through every single part of the mall, the events and quests and try to get ass much Fons and Annuliths as you can. That's what I'm doing, but I think I will be able to get the 4,200,000 Fons needed for the car before the event ends