Pcschematics.com and the MOBOT Motherboard Manuals Archive provide downloadable documentation for various motherboard models. Their collections include boards in purchased computers or boards that manufacturers do not actively support. Manufacturer documentation normally includes a diagram of the component and connectors on that board.
The challenging part of finding a motherboard diagram is trying to figure out which manufacturer or model number is in a computer. Some manufacturers print that information on the board, but many do not. Utility software, such as CPU-Z, can determine the manufacturer and model from the BIOS.
PC motherboard components use standard connectors and form factors, so generalized diagrams are useful when configuring a system or diagnosing problems. Build-Your-Own-Computer.net has a well-labeled generic motherboard diagram.