I have a Tamiya Mad Fighter - see attached

a) the wheels (tires ) - especially the rear ones have got worn out - where can
    I get a new set ? and what to specify when searching ??

b) I have put in a new Lipo pack and I find that though the car runs well ( and longer)
    it seems to be loosing traction . The pack is lighter than the previous Nimh one -
    is this a common issue faced by car guys who use lipos - and what should I do ??
    put some lead in the back ?

thanks for your time guys

You can buy any "1/10 buggy tire" and it should fit on directly. You can measure the inner dia of the tire and buy tires accordingly.

Regarding the handling issue, the car comes to life with a lipo battery, and for a car like this, to be honest this is not meant to handle well, it is a car to showcase, its a tamiya classic, it would not handle well when you take the performance beyond a certain point. Its not with this car only, the Tamiya lunch Box and a lot of other cars do behave the same way.
My suggestion , if you want performance  or handling get a new car, may be from HK. In case you love this car and have some memories with it, and want to use just this car, learn to drive it this way. Max you can do is get some new tires, they may help you a bit.
Hope it helps.

Thanks Himank - pearls of wisdom indeed .

Assuming I was in the market for a new car - which would you suggest ??

want to spend max  10K incl postage - preferably without radio and similar size

Hobbyking Zindabad, i myself recently ordered one costed 15k  (incld 2.4. tx rx 2x5000mah lipo chrgr etc)
you can get it here

http://www.hobbyking.com/hobbyking/store/__18737__Turnigy_1_10_Brushless_2WD_Desert_Racing_Buggy_ARR.html

Like you said, some lead weights would help. The added power could certainly be adding to difficulties.

The HK cars seem to work well. Take your time and make sure everything is assembled before you start driving it though. The ESC on the car mentioned above is very minimal. 35A is pushing the limits. Be ready to replace the ESC if needed. You will obviously need to stock up on a few spares since waiting for spares to be shipped from HK must be a pain.

Back to making your Mad Fighter better, softer rear shocks will help weight transfer and provide more traction(But I guess you have friction dampers and tuning them won't yield many results). New tires will most certainly help. Apart from that, Himank said it all.