Ask them for "mobile ke circuit saaf karne vaalaa liquid". they may hesitate first in giving you that as them may think you too are from some mobile shop, but be kind and the heaven may flourish with the fruits of kindness
Nail polish remover is acetone, which is harsh solvent. It may slightly damage component it falls on. Isopropyl alcohol (or isopropanol) is much gentel on surfaces and is ideal for cleaning electronics.
You can use cotton cloth, lint free cloth (the cloth used for cleaning spectacles is ideal for the purpose), of cotton swab (cotton ear buds), but do not use cotton balls as they leave a lot of micro fibers oon the surface which may lower TIM's heat transferring capacity and may make curing time longer (Arctic Silver 5 already has 200 hrs as curing time).