It’s all pretty easy in Uruguay. Maybe centralisation is bad sometimes but it does make things super convenient.
Tickets for most concerts and indeed for many carnival events can be done through box offices and two bill-paying chains called Abitab and Red Pagos/RED UTS.
A word of warning. All tickets need to be bought in cash and in a store (no online service yet).
You’re in luck. Abitabs are everywhere!
You can go and check their webpage if you read Spanish but it’s really not necessary. Abitabs are as ubiquitous to Uruguayan cities and Montevidean neighbourhoods. There are twelve in Ciudad Vieja. Thirteen in Pocitos.
There are a similar number if not more of Red Pagos however not all of them sell tickets (check that they offer the Red UTS ticket service). Slightly more difficult to find as they are usually part of another shop such as a Cambio (money exchange), look out for the green banner.
Buying tickets for carnival
Abitab sells tickets for the main carnival events such as the parades, the Llamadas and several of the larger tablados (in Malvin and Prado).