We usually use larger pistons that are procured through various means, relative to the pricing:
1. They may be fashioned by the machinist, they are cheap but they are hardly as precisely made as factory made and thus may erode the sleeve and weaken the integrity of the engine. Its a hit or a miss.
2. A lot of mechanics import their pistons from China. They are usually cheap, have huge profit margins for them and are generally of satisfactory quality.
3. Original Japanese imports are usually made on a specific order. Its the most expensive means but the quality of the pistons, rings, recesses is much higher quality with superior workmanship.
4. Entire engines can be procured from FATA where they are smuggled in from Afghanistan. Among them, damaged ones may be cannibalized for their their parts. Whole engines also sell cheap and are sometimes mounted on a Honda 125 frame with added brace and what is called the "Heavy Bike Kit", basically new tank as well as mounts for your 125 that make it look like a sports bike. However, using an illegal engine makes your bike street illegal and it may be confiscated but people sometimes go ahead regardless, you can get the Honda 250cc engine for about PKR 30,000 so its easy to see the charm.
Example: