You need to look at the existing power supply, or the plate on the laptop. It will give the voltage and amps output by the power supply. You need to get another one with the same (or very similar) voltage and amps. Also the polarity is important. This is whether the centre or outer connection has the positive supply.
Obviously the best solution is to buy an original replacement part from the laptop manufacturer.
An alternative is to buy a universal power supply, such as this which, will enable the correct voltage to be selected from a range and will have a variety of connections to fit most laptops. Like this and similar products: http://www.misco.co.uk/applications/SearchTools/item-details.asp?EdpNo=144797&am...
Failing that, you should be able to get an original replacement from the USA. The Targa range were sold more extensively there. They were mostly sold here as a loss leader by Lidl supermarkets. Try searching for ‘UPS targa laptop’ in Google but don’t confine the search to the UK. Obviously the postage might be a problem though.
Having set me off browsing through the Misco and Novatech Websites, I found that Misco have a special site to identify the replacement power supplies (and batteries) for laptops. Just enter your laptop model on this link:
Bye-the-way, can you stop asking this type of question … it’s costing me a fortune. Last time, once I was looking through the Novatech website, I ended up spending about £500 and this time I’ve found something in Misco for £130 that I know I’ll be ordering before the day is over!