If you look on your mobo, power to it is usually on the opposite side of where the component plugins are, such as your network/USB/Headphone etc. Usually near the cpu is where the power to that one goes. ETC, google your mobo and you should find a diagram. If you do work on your mobo, make sure your working in a static free area and rub the power supply first to ground.
Cheap enclosures can be found at microcenter if you live close to one, south mpls. You can find them for about $5, nothing fancy but to get the job done through USB2.0. Just have to know how old your hard drive is, if its the older ones, its a bit tricky to find IDE’s. You will find more SATA type connections. Not sure if we can post links but check this. HDD enclosures
Bestbuy costs for computer parts is about an arm and leg.