If you hadn't got one with your Sony HT, you'll have to buy one in any electronics shop.
their range starts from around 1k. There are some online stores too.
What type of outputs your HTIB has? If it has HDMI out then use HDMI cable. If no HDMI then check in the order of component, composite. HDMI cables are available in any electronics store or in ebay. No need for going expensive cables. A 150 bucks cable from ebay will do the work fine. I'm using these inexpensive cables with my BD Player, HTPC and HD DTH and they work fine. If you need branded cables then look for belkin, prolink etc.