Is there a good resource for ACARS messages?
Unfortunately the term ACARS is one of the most misunderstood and misused terms in aviation. ACARS - Aircraft Communications Addressing and Reporting System - was created as an end-to-end datalink between aircraft and the airline dispatchers. It provided basic text data using the ARINC Specification 618-8. (Unfortunately not free.)  

As datalink functionality has grown, ACARS has been modified and patched and added to to support these new functions. What is defined in A618 is now classified in the datalink world as AOC - Airline Operational Communications. AOC messages can be found in ARINC 633-3.  

The issue with that is that it is communicating with the dispatcher not ATC. That falls under the title ATS - Air Traffic Services. Early versions of this will be found in ARINC 623-3. That has mostly been replaced by newer applications defined in ARINC 622-5 which supports bit-oriented protocol (over the older character based protocols.)   

In the last 20 years or so, there has been a plan to provide ATS globally. The European industry which wasn't tied as closely to the ARINC systems as the US took a more more clean sheet approach referred to as ATN - Aircraft Telecommunications Network which isn't interoperable with the ARINC standards. So the industry now supports a mix of infrastructure. As it has moved towards global capability, the standards are migrating into the ICAO ecosystem.  

So the best reference available to find answers to almost any aviation datalink related question is the Global Operational Data Link Document (GOLD) published by ICAO.