I don't know if you have a habitat for humanity anywhere near, but ours has a regional store of all kinds of donated building materials new and used that you can buy to help support them. Kitchen cabinets , carpet, lumber, insulation, etc. Bargain prices!! Craigslist might be a source also.
Are you going to permanently close the garage door up? If so, I'd worry about getting the room sealed up first (moisture makes musty smelling fabric), getting heat out there and plenty of outlets, a cable line for tv and computer, a phone line. Try to think of anything this room might be used for down the road and put the necessities in now. Tables, storage and all that kind of stuff can be added as time goes by, but the stuctural stuff needs to be done up front! Its really exciting to be getting a new room, just take your time so you don't also get a case of the "I wish I would have...". My sewing room is the size of an extra deep one car garage. Some of my storage cabinets are the only things that cannot be moved. Mine was/is the downstairs family room and was done when I bought the place. I can honestly say my biggest complaint is the lack of outlets (5 in the entire space)
You are so lucky :mrgreen: :mrgreen: Have fun planning :D