Is there any way to have a Decanter of Endless Water pour out other drinkable liquids? If so, how could I get one? It seems reasonable to me as a possibility.
It's Up To Your DM
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.
Not quite, the closest item would be the Alchemy Jug.
There is nothing in the way of detailed lore behind the Decanter of Endless Water, so it is entirely up to the DM what this looks like in their world - they may choose to create a Decanter of Endless Whisky if they wish. But, there does exist a canonical alternative.
The Alchemy Jug lacks the volume capabilities of the Decanter of endless water, but makes up for it in diversity.
The Alchemy Jug can produce acid, poison, beer, honey, mayonnaise, oil, vinegar, salt water, fresh water, and wine, at a rate of 2 gallons per minute in varying quantities, with a limit of 1 type of liquid per day.
Yes, it can make mayonnaise.
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.
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.
For lore/historical reference, this is similar to the Everfull Mug from 3.5e's Magic Item Compendium, p.160, which was possibly related to AD&D's Mug of Plenty (Encyclopedia Magica).