Depends on you, your budget and requirements, that which kind of modifications you want; cosmetic, interior / exterior, performance type (like playing with the engine, exhaust, etc.), etc.
When you enter Hall Road from Mall Road / Regal Chowk side, there are some shops there which deal with sound systems.
You may go and check some shops there. Choose the shop / system which matches your budget requirements.
Generally speaking, systems from International known brands offer good quality, and accordingly, are expensive.
