I can’t find a real subpackage for an abstract class JModel (platform 12.1: http://api.joomla.org/Joomla-Platform/Application/JModel.html).
There is pointed:
Class JObject (Subpackage Base)
Abstract class JModel (Subpackage Application)
I don’t see it in a packages tree. I don’t see Base->JObject, nor Application->JModel, nor Object -> JObject -> JModel. I literally have no any idea where it can be found…
In 2.5 you should use the JLegacyModel class which Valentin mentioned. The base JModel class is in the legacy folder while the subclasses are in the cms folder. This will let you switch to J3 when you want.
libraries/legacy/model/model.php