Hello, brother. As a short answer to your question: "yes".
A good deal of consideration has to go into your choice of tire size. If you're planning on using REALLY big tyres (upwards of 33 inches), you're going to have to change the differentials (specifically, the guchcha, to get the gear ratios right). If you don't do this, it's quite likely your vehicle will end up going 0-100 in an hour, rather than in seconds.
Secondly, you'll probably have to consider a suspension lift, so as to fit the tires under the body.
Third, you MAY have to chop the body to accommodate oversized tires within the wheel arches. This would again, depend upon the size of tire you choose.
Fourth, is the cost-effect. A decent set of (used) oversized tires in the +33 inch range is likely to set you back, a minimum of Rs. 100,000/-, including rims.
Fifth, you'll have to consider the fact that since it MAY involve a great deal of work to the underbelly of your vehicle, you'll have to find the right specialists to get the work done for you.
However, all of these things are ultimately doable. Which brings me back to the short answer to your question: "yes, monster tires CAN be fitted to an '88 Pajero".
It will all depend on how "hungry" you are to change the look of your vehicle, how much time and money and effort you are willing to put in, and what you want your vehicle to end up looking like.
Sorry for not being able to answer your question as to where you can find such stuff in Karachi. I'm from Islamabad, and haven't got the slightest idea about your city.
Best of luck.