I want to meditate on Lord Vishnu and Lord Shiva. But I have read at many places on the internet that before chanting a mantra one must receive it from a guru. What another option do I have? I currently chant vishnu sahasranamam daily.
You NEVER needed a Guru for obtaining any knowledge. Who was Valmiki's Guru??? Who explained him the biography of Rama? NONE.
Who was Eklavya's Guru??? You may say Dronacharya, but no he was not. Eklavya considered him as a Guru when he watched him teach the pandavas and kauravas.
When you meditate and you are lost from materialistic world, you gain knowledge without you realizing it.
Recite - OM NAMA SHIVAY to make Shiva come to you and recite Acyutananda Govinda to bring Vishnu to you. And remember both are one and same.