Question: Is there any way to have a [Decanter of Endless Water](https://www.dndbeyond.com/magic-items/decanter-of-endless-water) pour out other drinkable liquids? If so, how could I get one? It seems reasonable to me as a possibility. If not how might you go about modifying a decanter of endless water to do so?

Answer: 
It's Up To Your DM
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Technically, the rulebooks have no Decanter of Endless (Insert any liquid here) among the plethora of items Wizards presents us with, however that doesn't mean that is impossible. In the DMG, in Chapter 9: Dungeon Master's Workshop on page 284, we are presented with the "Modifying An Item" section, which, since I am not going to quote the whole section, suggests three types of changes: Cosmetic, Property, and Fusing Items. I'd say your thing would fit under the property section, where it gives examples like changing an object's damage type or effect. As a player, this is off-limits. Speak to your DM, and they might allow it.**The Rules are guidelines, rather than rules. It's up to the individual DM to decide how much they want to stick to them. ;)**

Otherwise I'd agree with Thomas and say Alchemy Jug is your best bet.
The Alchemy Jug (as mentioned above) would be closest, but also note that Hoard of the Dragon Queen p.74 contains a Tankard of Plenty.

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> The Golden Tankard takes its name from a magic item that Raggnar found years ago: a golden stein decorated with dancing dwarves and grain patterns. This is a tankard of plenty. Speaking the command word (“Illefarn”) while grasping the handle fills the tankard with three pints of rich dwarven ale. This power can be used up to three times per day.
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For lore/historical reference, this is similar to the Everfull Mug from 3.5e's Magic Item Compendium, p.160, which was possibly related to AD&D's Mug of Plenty (Encyclopedia Magica).