Q. From where can I get wireless slave flash?
Umm...I don't know about your city, but in Karachi they are widely available in Regal. For online sources IMO i would suggest you pay a visit to
Cheap Digital Camera Accessories, Photography Equipment And Accessories - Free Shipping - DealExtreme
They have chinese brands and they ship to Pakistan free of cost. Payment method is online Credit Card. A couple of PW'ers have used it, so can be recommended. I too hope to use it for a flashgun, iA.
Q. I want to improve the lightning on the subjects. Which brand is good?
will it help?
For proper lighting, portrait shoots and macro photography built-in flash wont work (i hate it btw) and never use it. Its very harsh. External sources like strobes are recommended. Either you can make your own strobe (madhouse had a thread where he used an energy saver and some cardboard) or buy a flashgun for this purpose. I suggest try the DIY if it doesnt meet your requirement, go for a proper flashgun.
There are various brands in the market. First are Nikon and Canon (very expensive), then are brands like Nissin (affordable and reliable) and thirdly there are 3rd party Chinese brands like Yongnou (very cheap). I personally suggest get your hands on a used one to try from a friend for some time. If it fulfills your need, get Nissin, if you want to go for cheaper option, go for Yongnou, more cheaper do the DIY. 
(I might go for Yongnou. Recommended models are YN-465 and YN-560 and will have to get a wireless transmitter for that)
Given that your P&S has a hot shoe, otherwise only YN-465 would work as it has TTL.
IMO it should help. Trying it for yourself is the BEST option. For more information about flashguns and their compatibility, consult Khurram_aziz bhai. He is more into Portrait and Lighting techniques.
I am sorry but my replies are always very long. I hope to clear everything 
Q. Is there any other thing which can be done except upgrading ?
Umm....P&S would work great if it has Manual mode and i'm assuming urs has, so it would work great. Hot Shoe would be needed for wireless flash transmitter to work. If Photography is your passion and want to venture in indoor and portraits, get a decent DSLR.
Dont want to upgrade? Try the Softbox DIY. I will try it myself too, given that i get time off from my busy uni schedule 
Ill try to find the link and post it here. Google for home-made DIYs for softboxes. It's just that they are not THAT effective and are no portable.
I hope i answered all the questions well
basically I want something for portraits. I usually shoot in 600-800ISO range in which the little pns shows noise. I want to bring down the ISO till 400max without using tripod. Any suggestions are welcome
Yup P&S are not that great in Low light. Using tripods for portraits is not that great as your shutter speed should be around 1/60th or more to avoid blur. Lowering the ISO means u should have DECENT lighting in the room. A powerful soft-box would help a LOT.
Use the camera by hand, grab a soft-box, decently place it in front of the subject.
Keep ISO as per ur request at 400
Keep shutter speed around 1/60th or 1 step lower if still need more light
Keep aperture biggest in case of low-light. if light is sufficient then 7 is great.
Sorry for the extra long reply. 
If you have still any questions or doubts, feel free to ask. 