I doubt that you will find prepared plans that are useful for your project. Adding living space above the garage requires specific structural considerations that must be incorporated into the plans, including foundation, framing, roofing, snow load,and access. You will also need HVAC and electrical details. These plans need to be drafted and usually require sign-off by an engineer or architect in order to be approved by the building department for a permit. Because remodeling is so specific to the existing structure, its not like building a whole house or garage, where you can obtain plans off-the-shelf and modify them.
I have prepared such plans and had them reviewed and certified by an engineer; and of course you can contract for plans from scratch with an engineer, architect or registered designer. If you can document the foundation details, provide a framing plan, calculate loads and comply with local codes, no problem; otherwise I recommend you start with professional assistance. Call your building department for assistance and information on permit and plan requirements. This will give you a good start to know what is needed. They may also be able to direct you to local professionals that are experienced in plan preparation and submittal for permits. This does not mean you can't build yourself, but starting with good professional plans will make the work, and any subcontract bids, easier.