Sometimes the buy option is simply absent on a game you know has one, and the cause is usually the operator rather than the provider. Casinos can switch the feature off per game or per market through the integration settings, and a few do it without mentioning it anywhere. The casino overview at https://thebonusbuyslots.com marks which operators have the feature live across their lobby, which saved me from depositing at one that had disabled it entirely. Support chat will usually confirm this if you ask directly.