Online documentation
The online documentation contains a lot of information about WebTest with numerous examples. The documentation of the steps is generated directly from the Java sources and therefore always up to date.
Additionally you can look at WebTest's Self Test Report to see how steps are used.
Mailing list
WebTest's Mailing list is the right place for all questions concerning WebTest. Most of the time you will get a very helpful answer within a short time.Paid support
You probably don't need it because you can get good and free support from the mailing list!
Otherwise, nearly every qualified Java developer with web knowledge should be able to understand how WebTest works within a short time and could be able to use and customize it as he/she needs.
Finally you may be interested in support directly from WebTest committers if you want to quickly learn web testing best practices from people having years of experience in this domain or if you have special needs. In this case, please contact them directly.
WebTestRecorder