Another way that the (White-black archetype) can ramp mana to pay for decadence is using mana rocks. Several sets have a common or uncommon variant on Manalith with upside, so a version of that which is particularly good at paying for decadence seems like a sensible thing.

Decadence itself only needs colourless mana, so the second ability here could just add . But I like making it add because that makes the rules text hint at what you might like to do with it (since decadence is a mechanic), while still letting you use it for other things like instants if you want.