A camera store I went to recently in SF had a really inexpensive filter kits with ND4, ND6, & ND8 for $55-75 USD. Perfect for breaking or stacking and not ever being able to separate them again. Haha. I use them all the time, I even bought step down rings so they fit my 72mm & 67mm lenses.

Hope you post many more long exposure images, good luck!