I wasn't sure if AE Services were still in operation. I have a lovely thick strap I bought from them, called a Persuader. I don't self-spank (although I used to as a teenager) but I'm sure it would be perfect for the job. I also have a long-handled clothes-bruch that my Dad used on me that was my favourite implement when I did used to self-spank.
One of my "pupils", for whom I act as cyber tutor when he can't visit me, has a Yahoo Group devoted to self-spanking:
http://groups.yahoo.com/group/selfimposeddiscipline/
where you'll find plenty of advice about implements and techniques.
Tim