Gamingetc.com also sells singles
@NidoJosh - I recommend using the sites above to find common singles (i.e. cards released in normal sets), but when it comes to finding promos or rare foreign cards, you should move to Ebay (or possibly the trading/selling/buying forums here), as, not only will you be able to find more, but you will get better deals.
P.S. If you are considering a Nidoking collection, and you do try and get all the Japanese versions, watch out for this bad boy.
Giovanni's Nidoking seems to be a very hard card to find.
P.S.S. Use Bulbapedia to find lists of all (well, 99.999%) of the versions of cards (exclusive to English and Japanese for the most part). Just search the Pokemon name with TCG after it, and it'll give you a list of all the versions.
Here's an example for Nidoking:
~click~