Do you mean which type (CPL or linear), which coating or which diameter? Please specify and also check threads in other sections. Filters are a general topic, nothing related to Nikon. The search function will help.
:I got a Kenko CPL filter from John 3:16, cost abt $68, but of course the amount you pay is the quality you get. Mine was the Pro 1D filter which reduces the f/stop only by 1. While other CPL usually reduces by 2-3stops.
Do some search on the type of CPL filter you are looking for.