Start by checking the coolant level when the engine is cold. If that's ok it could be a bad thermostat, clogged heater core, bad blend door motor, and a few other things. Use this link to find a good shop near you.
https://repairpal.com/repair-shops
Alex
RepairPal Certified shops can diagnose and repair your 2008 BMW 535i. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.