I use the standard Expodisc on a Nikon D300 and it does a very good job of making the color balance very neutral. Unexpectedly, I often find myself warming the image just a tad from there because that's what I see with my naked eye in most situations. I personally find it more useful in scenes with too much blue or green...say when there is a lot of fluorescent lighting for example.
In hindsight I should probably have gotten the portrait-version of the Expodisc, which probably renders a color balance closer to my preference.
The point of the story is that "perfectly neutral" white balance wasn't really what I needed, but it was what I thought I wanted at the time. (with my Fuji X100 I don't think I ever need one - the auto WB is uncanny in it's precision!)
My recommendation is that you perhaps first try with simple white balance card, a white piece of paper or something like that, until you have a good feel for what kind of balance that is your preference.
There are several less expensive alternatives than the (quite expensive) Expodisc. Lallycap is one example..looks like a sock for your lens. You can also buy white balance lenscaps which seems pretty clever, until you realize that you need one for every diameter of lens you use. They all work as intended, but how good they are for you, only you can decide.
If you go to your camera store of choice, bringing an object you have photographed several times, I'm sure you can try several types out. Compare them to the very cheapest version - the white balance card - and see what you prefer.
Should you land on Expodisc as your preferred option, make sure you get the largest one, so that it can be used with all lenses. If you get a 52mm it will be awkward to use with a 77mm diameter lens.