In addition to Mike's suggestion of B and H (where you can find some good deals on Tiffen filters), I would suggest Cokin filters, especially if you don't want to spend the extra mone on a Tiffen filter. I found one here: http://www.adorama.com/CKZ153.html?gclid=COuu2-iss7gCFWQ6Qgod5FgA3A.
Also, LEE makes some less durable polyester 4×4 filters if you're really looking for a less expensive deal: http://www.bhphotovideo.com/c/product/102523-REG/LEE_Filters_6NDP_4x4_Neutral_Density_ND.html.
Even so, I would go with the Cokin or Tiffen filters. I know both of those brands to be fairly reliable. Hope that helps!