Depth of field adaptors are devices you attach to your video camera to help provide the look and shallow depth of field capable with 35mm film. Generally these can get pretty pricey, but you can save a lot of money by building your own. It seems a little daunting but it's actually not as complicated as you might think.
The video above demonstrates a more complex, high quality method where you purchase and assemble the different parts required to produce the adaptor but make the GG holder (the small, round piece that holds the focusing screen where the image is projected) out of copper. This is really the ideal way to do it and it's not that complicated, but there are easier versions, like this one, that uses clay to keep it in place. You can pretty much use anything that will hold it in place, but you'll want to consider the durability of the material as it is very important the focusing screen doesn't move or fall out. If you don't want to make one, you can always buy a GG holder as well. You may want to do this as purchasing one that vibrates will improve the resulting image.
Aside from your GG holder, you just need cheap camera extension tubes, a spacer ring, a macro lens, a step-up ring, and a circular polarising filter. You can find the entire parts list, plus in-depth instructions, here.
Tutorials [Daniels GG-Holders for 35mm DoF Adapters]