I personally haven't changed one myself, but they are relatively easy if you can find a machine shop to press them in. Other members have managed to do it without the machine shop but it took plenty of elbow grease and some clever thinking. To get the ball joints done, it would entail removing the part that it's fixed onto. I believe it to be the spindle. You would need to remove it and take it to the machine shop to press the old one out and install the new one in. Then go back and reverse the uninstallation procedure. And of course, torque the bolts to specs.
The early model LS's had a problem with ball joints and Ford had a recall for them due to improper torquing of the bolts. Silly question, but ask the owner if he's taken his in for the recall notice.
As for purchasing, there's a member here that sells them by the name of euromarque. I believe he runs some of his items on ebay, but if you mention it to him that you're from here, he may cut you a little break. Maybe since his prices have come down a bit.
I hope this was of help to you.