EXTRA ND FILTER for Stock Lens?
 

I want to achieve shallow DOF with the stock lens, I can get pretty good results with keeping the iris open using the two builtin ND filters, zooming in and using the macro to focus and the shutter speeds to expose. In most daylight conditions I can achieve max shallow DOF with the shutter set at 1/500th. I want to get a filter to put over the stock lens to filter the light so I can maybe just use it at 1/100th. Please don't criticize my methodology, as nuts as it sounds, it actually works. Where can I find such a product to put over the stock lens so I don't have to close the iris whatsoever/have a inappropriately high shutter speed/angle. I want an ND filter to phyiscally put over the lens. Do any of you have experience with this? If so where can I find?

There are two options to add an extra ND filter. Through an extra filter which you screw on the Stock Lens, or adding a ND filter when you have a MatteBox.
With a MatteBox you have of course much more options to go.

This link at B&H gives the 3 most commonly used Neutral Density (ND) filters, .3, .6 and .9:
http://www.bhphotovideo.com/bnh/cont....x=0&image.y=0

Search B&H for "82mm Neutral Density" -- without the word Tiffen -- and you will get a lot more options by different companies, some in combination with another filter (such as a warming filter), and different prices.

These filters screw onto the lens.

A polarizer can also help reduce the light input if a polarizer will work for the shot -- about 1.3 stops reduction in light.