There are quite a few threads on this forum that describe the process in detail along with pictures. My only advice would be that once you get the knuckles off is to take the ball joints to a local shop to have them pressed off and the new ones in. It is alot easier and most shops only charge a nominal fee. The same would go for when you are re-installing the seal. There are instructions on this site to build your own tool but I found it still didn't work right. It was easier just to have a local guy do it for $10 that has the tool to do it.
As far as where to find the joints, there are a ton of sites that have them. Their prices fluctuate by a few dollars here and there but they are mostly all the same.
BALL JOINT | CarPartsCloseOuts.com
Shop for Ford Ball Joint at 1A Auto
Oh and when you put on the new ball joints the top one won't fit with the zirk attached. You have to leave in the screw and put the zirk in when you need to fill the unit.