Where can I buy a 3-stop and 9-stop filter, 67mm (18-135mm) and 77mm (for a sigma 10-20)?

I did a quick search and didn't come up with much. Bhphoto has a gazillion filters and their search function blows.

bhphoto has them, the 9 stop hoya is called an "x400" so if you search for that you'll find it. At B&H go in to their filter section and use the filters to narrow it down to what you want (specify type of filter, size, etc).

2filter.com

my Hoya 9 stop HMC 77mm will be for sale as soon as HiTech puts their 9 stop rectangular filters on their website which is supposed to be real soon.

Not sure what your time line is though.

I will also be selling my Hoya 77mm NDx4 (2 stop)

i have a promaster 4X 77mm that I was going to put up for sale. $15 if you want it..its like new since I never really used it lol