I bought this $40 77mm ND1000 filter through ebay ('bondscamera', Hong Kong), brand name on the filter is Camdioxhttp://www.ebay.com/itm/350576100078
Also this $21 Cokin P size graduated and ND filter set with holder and adapters through ebay ('mambate', New Jersey)http://www.ebay.com/itm/281097436699
Both work well for me and my uses.....YMMV. If you just want to get something decent that is actually available so you can play around with it then I would recommend both of these. Purists may cringe but they are welcome to PM me so I can give them my mailing address for that elusive Lee Big Stopper and other top shelf items they want to send me.
I was recently in my local BestBuy and found a 77mm CPL and 77mm ND3 that included a 72-77 adapter. Reasonable price and they work on all of my larger lenses including the 300/4L and 400/5.6 and most happily the Tokina 11-16/2.8.
I would strongly recommend against a variable ND, there may be some that do not exhibit the cross effect at higher density levels but the vast majority do, particularly any of the lower priced ones.