There are plenty of companies that provide transfers to and from Delhi airport. Delhi Taxi Service and Taxiwala are two of the main taxi companies operating in the city. You can also ask your hotel to recommend a company they use on regular basis.
I would however allow extra time to ensure you arrive at the airport early, because even if a taxi driver is reliable one can never predict how bad the traffic might be on any given day in Delhi.
Good luck. Hope this has been helpful to you.