You sell idols you get from events etc for drinks and slowly save up. Also some SRs from the 10% etc tickets still sell alright and the new rares you can usually sell for few drinks since people buy them for the event boost. Back when I was playing I was able to get about 20 drinks a week from trading etc.
Also try getting top7000 in event for the 5% chance to get the event SR, if you do that's like 300 drinks. I managed to get one from 5% ticket and one from 10% ticket (top3000)
But generally if you get lot of the event coins and get SR+ of the one from the coins, that's usually like 15 drinks.
Those 500 ones for non+ are usually only the top cost and the newest released cards, if you wait a bit their price will drop to lower once new batch of idols get released or even better new cost number. After a while you can usually get SR+ for like 700ish (350 each).
Having more than few such cards is really hard for free.