Many bonsai growers love thick trunk and quickly tapering bonsai. They often write and us for a technique on how to create such a design. Unfortunately, there are no quick ways to obtain a heavy and tapered trunk.
One, is to search through hundreds of pre-bonsai to select several that 
have the potential to become a good tapered trunk bonsai. Most pre-bonsai material doesnot have the potential for such styling.
Two, reduction or chops are critical to developing the taper that is desired. Basicallya chop is made at a point where transitioning to a branch allows the trunk to taper. Sometimes several reductions are necessary over a period of years to achievethe desired taper. Reductions if done on the back of the tree make the reduction cut less obvious. An excellent piece of pre-bonsai material may allow 3 reduction cuts to be done at one step thus reducing the number of years needed to create a good bonsai design. 
Three, grafting of aerials to the trunk  
Four, a proper transition to a believable apex would need to exist on the tree as 
growing a totally new one would take some years.
So, spending a lot of time in dealers who have likely candidates would allow one to find candidates that would make great starting points
would certainly help. Or finding one that has been pre-bonsaied by a 
dealer would put you on the right track.