I guess you already know that ZhongGuanCun (中关村) in Peking (北京) is the largest collection of shoppping centers selling computers and electronics stuff.
If you need something for your computer, I think you will find at least 1,000 shops/counters selling what you want. The hard part is getting what you want at the price you don't regret later.
The trick is to buy only things you have researched thoroughly. Use a place like eBay to find out the lowest market price.
Buying something on impulse will always end up with regrets, unless you are a trained negotiator.
ZhongGuanCun is not one mall. It is a whole city of malls. I don't know how many there are. I went to only a couple of buildings. Basically they are all the same. I guess in lieu of marketing to differentiate, each just bank on fleecing one unsuspecting customer a day.
I went to look for a notebook battery. Surprisingly this was one item that was not really that popular. If you need a battery and want to save time, follow these steps:
a. Go to HaiLong building (海龙大厦). This is the mall directly outside the west exit of the ZhongGuanCun subway station. This is on line 4, a brand new line. Click the following air photo to see details. Go to Google Earth/Maps to see readers' pictures of the building. Just note that most of the positions of this building on Google Earth are wrong. The location on Google maps is here but unfortunately the map and photos don't quite match.
b. Go to the fourth floor and look for the HoneStar Technologies counter at #4075. The numbers are on a license plate hanging off the ceiling. #4075 is furthest way from the escalators. It is manned by two young ladies. If you have limited time in Peking or you think your battery is a rare one, contact them before hand.
Email: email@example.com, QQ instant messaging: 437105767, mobile: +86-13552906007.
English is barely spoken. The best way to contact is to use QQ if you know how to install and use the software. Otherwise email and then follow up a day later by phone to ask Miss Liang to check her mail.
The building opposite Hai Long to the east is KeMao (科贸中心). There is a pedestrian bridge from the second floor of HaiLong going across to KeMao. KeMao is another mall of the same, hundreds of counters selling computers and electronics. But there is also a large supermarket (Wu Mart) and a KFC.
Hope you get what you want!