There is a brushpack available on this site I do believe.
If not go to Deviantart.com and browse resources/photoshop brushes or something. The brushes you use depends on the style of signature you are going for. Deviantart has a wide selection of brushes made by some very talented people - have a look for grunge/abstract brushes of some sort, they are quite popular.
You can also make your own brushes ^_^