Generally, it is far more expensive to restore a vehicle than it is to buy one already restored. People restore cars for personal enjoyment and they almost never recover the cost of their time and materials in the work. As an investment, older cars usually hold their value or gradually appreciate over time, depending upon the what vehicle styles are in vogue.
Once you have decided upon the make and model vehicle you are interested in as well as as its state of repair (or disrepair), you need to get one. There is no one best place to find the car of your dreams. eBay is an excellent place to start for vehicles in any condition. However, if you find your perfect car in the USA, do NOT buy it unless the car comes with a clear title. Cars without titles cannot leave the USA.
Often the best place to find a specific type of vehicle is to join an on-line discussion forum specializing in your vehicle. You can find these by just starting with a search like vehicle forum, where "vehicle" is the make and/or model of your vehicle. Other places to look are general automotive web sites like these: