You can slowly upgrade the traits of the npc girls, using this potion.
Downside being you can only buy one at the end of the day and its rng so it might not even show up.
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The potions from limited sales increase in cost every time you buy it, so it's not a long term solution.
Best way to get high tier girls is breeding them with gold tier brothel clients (unlock VVIP room and turn condom usage off for gold). Gold tier clients have
all positive traits and they pass these down to the children (who are always girls). Then breed the grown up children with gold clients again. Keep doing this and pretty soon you will have girls that has 100% conception chance, 5 maintenance (lowest possible), 3 simultaneous pregnancies (highest possible), any number of maximum pregnancies (depending on how many times you repeat the process).
But gold tier clients are rare, unreliable (they may ask for paizuri or anal sex, and they may cum outside). Also brothel reputation is slow to build up. The solution is that the player is considered a gold tier client and can breed girls in the mansion. So buy the mansion (it may require the DLC), build a slave cage, then build a vip room and delivery room, will require about 60,000 gold. Then transfer and breed a girl, breed the daughter, then breed the granddaughter so on and on. Each generation will get better and better. After a girl gives birth send her to the main estate to breed with monsters.