Micheal, there is a file with the shapes. On a Mac (don't know about Windows), go to your VectorWorks folder in 'Applications' and open the following file path: Libraries/Defaults/Section - Markers. Open the file 'Section Markers.vwx". Go to the Resource Browser and look at the symbols in it.
If you want to change an existing symbol shape, select and edit it in the Browser, then save the file.
If you want to add a new one, duplicate one of the symbols in the Resource Browser, re-name it using the same naming convention, change it to your liking by editing the symbol, exit the symbol and Resource Browser, then save the file.
When you've done the above, open your drawing, draw a section line, and change one of the markers. You should see your new marker in the pop-down window that appears. Select it and you should be on your way!
Hope this helps!