Where can I buy and fliters for my SLR? (preferably online)?

I have a Nikon D5000 with a Sigma 10-200mm lens and I'm looking for a suitable and filter for it.

Ebay has great filters at great prices. If you don't like ebay you can always try amazon and overstock as well

When it comes to photographic gear, the only two places I use are a local camera shop and B&H Photo… Http://www.bhphotovideo.com/c/search?Ntt=nd+filter&ci=16773&N=4256189565&Ns=p_PRICE_2%7C0

You will need to look on the front of the lens to see what size filter you need. You will see xxΦ (52Φ, 58mmΦ, 72mm Φ, etcetera)… This is the filter/cap size for that particular lens.

I think that's a 67mm filter but check.

I like these guys. Genuine hoya and B+W at very good prices: http://maxsaver.net/Hoya-Neutral-Density.aspx