Last year I decided to purchase a spanking machine. There are two models that are readily available on-line -- the Robospanker and the Spankomatic II. The Robospanker is better known, due in part to its association with Howard Stern, but I ultimately chose the SOM II. It seemed to be more flexible in how it can be used. Although it is relatively expensive, the quality of the machine has shown the cost to be justified. It is extremely well-engineered, and it functions just as advertised. This includes the ability to give very forceful strokes with a variety of implements. Here's a link to the SOM II web site:
http://www.orgasmalley.com/som/index.htm. I've had no experience with a Robospanker and can't speak to its use.