I recently purchased some Lee ND and ND grad filters. I had a tough time finding them online, and finally searched around locally for a store that sold them. Best I can tell, Lee filters are not much of an online commodity, and are sold in retail stores most of the time. I do believe that Lee makes some of highest (if not the highest) quality filters available, and if you can find a local store, they are definitely worth it.
Here is a PDF that might be helpful:
Lee Camera Dealers
The story on Lee filters is a much more complex one than I originally noted above. I seem to have been lucky finding anything, particularly the foundation kit, locally. There isn't a single local store near where I live anymore that has any Lee filter equipment. I've turned to online sources, but they are nearly as dry. I managed to get a .3, .6, and .9 Soft GND set from B&H, and have found some filter adapter rings on Adorama. Generally speaking though, the most disappointing thing about Lee filters is the simple fact that they are not available...ANYWHERE. I've been on a waiting list for some filter slide guides and extension screws for my lee foundation kit for months now, with no hope in sight of actually getting any of them. I've been waiting to get some additional solid ND filters, but they also seem to be completely dry, with no date in sight for when they may be available.
There seems to be a couple online stores in the UK that have all Lee parts, but the cost is considerably higher than the already high cost here in the US, not counting any import fees. Very sad state of affairs with Lee filter equipment. There is no question Lee makes the best filters around, but without any of it being available for general purchase, their amazing quality is kind of moot. Perhaps an online petition to Lee to step up production and fill their supply lines would be useful.