For stamina and energy drinks real money is not that of a viable option for most people, otherwise you are paying like 1000 bucks for a single idol even (1 stam drink costs like $0.9 USD) xD
Probably getting those gatchas and step promos where you have increased chance of S rare might not be bad, I didn't buy those with real money but from the events and such what I got I already got 2 idols that I can sell for about 70-80 stam drinks (sold one so far to buy chibi akuma Miria for 60 and still have one - basically you want to get 20 cost idols and trade those, even the ones I got that didn't evne have voice sold very well since they have good stats but you can trade them for voiced idols easily).
Other slower ways can be the event cards, if you can get the good ones from the slot machine and the rankup, you can trade those after they open for sale, I for example traded my Asuka (one of the last event S Rares rewards) for 12 stam drinks and then few days later, I bought her for 2 energy back since I missed her xD
Other way is to use your free trade tickets to earn it (you get those from platinum gatchas), for example you can buy 2 regular S rares like the Miku one for 4 stam each, special training it and dump some levels into it (usually a stam drink is worth about 2mil money from what I observed) and sell the S rare + for 13. It will cost you 9 free trade tickets (2x3tickets to buy them and then 3tickets to sell) but you get 5 stam drinks.
But yeah, without real money its much harder but if you have something to spare if you can score a hit on the limited S rare like for example currently the Yui one, it sells even over 300 stam drinks (thats like 300 bucks worth of stam drinks) so if you buy 50 bucks worth of gatchas for the special Step gatcha where the chance to get her increases and all and get her, you are in huge profit.