Public transportation systems are complex systems. The consequences of real-world decisions can lead to high costs or job losses. Simulation models provide an understanding of complex systems, predict outcomes, or test different scenarios while avoiding cost risks. Maintenance and repair processes for public vehicles are intense. Therefore, spare parts supply is critical. There are two alternative replenishment policies in the system. These methods are tenders and direct supplies. Tenders are useful for purchasing large amounts of spare parts with a longer lead-time while direct supplies are useful for purchasing small amounts with shorter lead times. Stock management ensures that the right amount of stock is available at the right time and place. An excess or shortage of stocks leads to different negative consequences. In case of excess stock, holding costs increase and tie up capital. On the other hand, in cases of shortage, demands are not met, and sales, revenue, and customer losses occur. The main issue is the lack of clarity regarding the timing, optimal stock levels, and the procurement procedures to be employed to replenish spare parts. This problem is so severe that it takes over a hundred days for some buses to become operational again. This study aims to analyze the current system of the public transportation company, to simulate the system as close to reality as possible, to use a stock management policy that fits the company structure, to try different scenarios on spare parts supply methods, and as a result, to improve the existing complex system. A simulation model of the system was created via Arena Simulation Software and alternative supply policies were tried in different orders.