I just bought one second hand which is a manual one. I have to check the water and humidity levels and turn the eggs every eight hours (although I have heard about people turning them only a few times a week, and some not turning them at all and the chicks still hatched). To many people it would seem like a bit of a pain but I don't mind it. It is cheap to build your own manual incubator too.
Just get or make a box with a ledge around the edge. You will need a container in the bottom with water, so make sure the ledge is higher than the top of the water container.
Put some mesh to rest on the ledge (make sure its strong enough not to bend under the weight of the eggs or chicks, or strengthen it with a piece of wood going under the middle from one side to the other.
Attach a heat element obviously to provide the heat.
Get a thermostat - one that just plugs into the wall with another plug like the link below, and plug the heat element into the thermostat.
PLEASE NOTE: The link is for a product on the UK
Ebay site, and this particular product would not be suitable to use, as it only regulates temperatures up to 35 degrees C and chick eggs need 37.5 ideally. Unfortunately the one I know about which is suitable is not on the shops website.
http://www.ebay.co.uk/itm/TIMEGUARD...-HEATING-/260931741923?_trksid=p2054897.l4275
Set the thermostat and start turning the eggs every eight hours for 21 days, checking water levels every few days.
Find chicks.