First look through all your paperwork to see if there is any manufactures build sheet that identify the actual AC unit installed. Sometimes the sheet is attached to the inside of a cabinet or door, so look around.
If that fails, you can always go to the unit and remove the shroud. On the AC unit is an identification plate with all the info for that unit (make, model, amps, etc.). I strongly suggest if you reach this point to take a picture of that plate and keep it for future reference.
As for where to get a replacement, just about any RV supply house or center can get a generic, general or specific shroud to replace the one you have. many generic and general shrouds are designed to fit multiple models across several manufactures. Before ordering the shroud, make sure you understand the installation as different shrouds can use different methods. Some may be identified as fitting your unit but with a bit of research it could show that some slight alterations need to be made to fit exactly.
And you always have the option of taking it into a place and have them look and make recommendations. They may have several shrouds and can tell you which is easy for you to install yourself or if modifications can be a bear. It doesn't hurt to at least get a quote.