Amazon.com has quickly become my favorite, secret spot to buy things like spare USB extension cables, iPod cables, USB convertors, HDMI cables & convertors, and so on.
I used to use sites like Newegg.com, and that's a great site too, but you just can't beat the prices on Amazon.com. And no, I'm not being paid anything or getting any kind of reward for sending people their way.
I'll give you an example from a recent purchase I made. This is a DVI to HDMI convertor. You can use it to connect your PC to a TV, so you can use your nice big TV as a monitor:
Apple iPhone cables-- at Target they're $20. At Amazon they're as low as $1, free shipping.
The examples go on and on. For the most part, any cable or convertor I've ever looked for has been cheaper on Amazon than anywhere else, though, there is a little bit of a secret to the ordering process.
I usually don't just click "add to cart" from the main page. I usually click on the "XX new from $xx" link at the top of the page, and look at all of the available sellers.
Make sure you order from a seller in the US (or whatever country is closest to you). Some of the really cheap ones come from Hong Kong, and take literally 18 - 30 days to come in! I've had this happen to me before.
So when you order, spend the extra $0.50 and go with a closer seller.