Examples of thermal energy reservoirs (models) include rivers, large ponds, the atmosphere, the Sun, etc.
Sometimes phase change systems that work on heat transfer at constant temperature can also be considered thermal energy reservoirs. They just utilize the amount of heat provided to them in the form of latent heat and only undergo a phase change. And as we all know, during the phase change the temperature remains constant.
The thermal energy reservoirs usually have a very large mass/size, but it does not need to be always true. The thermal energy reservoir can also be of relatively smaller mass/size. For example, a CPU losing heat to the air in the room in which it is placed can also be considered a thermal energy reservoir.