3 stop or 6 stop really... depends how much light you want to cut out.
i ordered a 10 stop 77mm... and that was a mistake. my math was off i guess but its pretty ridiculous. i havent really used it at all and would love to swap it for a 6 stop.
as for where to buy, i've heard good reviews about
http://maxsaver.net/ and was about to place an order for some UVs and a CPL from them... pretty good savings.