Check this out, I came across this earlier today while going through my emails: Harbor Freight is selling a pneumatic planishing hammer for $120 ($30 off regular price). I don’t know much about it, but seems like a decent deal. I wish I had the room for one.. one of these days….
Here is the link: http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=94847
Now if you want to build it yourself, instructions to do so can be found here, good stuff: http://www.autobody101.com/articles/article.php?title=Home%20Made%20Planishing%20Hammer
Still, for the money, the Harbor Freight one seems decent, although I don’t know about the quality. If you have any experience with that particular planishing hammer, post about it in the forum.