If you're not in a super rush to get one, get it from Hvstar out of Hong Kong...it'll take about a week to arrive, but you'll save a bit of money. Try this one for your lens:
B+W 77mmKasemann MRC CPL
Hvstar's prices have gone up a little bit (not sure if its because of the exchange rate which is getting better, or because their sales have gone way up). Either way, they still have the best prices on B+W filters. The non-Kaesemann version of this filter is only about $10 less, so for the money you're better off with the linked version. However, whichever filter you buy, make sure that its the multi-coated version, designated with 'MRC'. The one's that you see for $70 are not MRC. If you want a good filter, you'll have to spend a bit more...