You can visit hrtc official website for bus booking. all types of buses available here according to your budget. you have other option like private buses or taxies also.
For delhi to Manali
you can take volvo from Kashmiri gate and it will cost you nearby 1.5k and it ia best option for travelling to Manali. Also it will take a overnight journey.
You can directly book bus tickets at the Kashmiri Gate in Delhi or Online tickets for buses available on Redbus,Goibibo and MakeMyTrip for better availability.