How does the sender obtain the receiver's public key? Cryptography process includes encryption and decryption process. In public key cryptography public key of receiver is being used for encryption.

I want to know how public key of receiver is sent to sender?
The sender receives the public key through some other (secure) channel.

The whole security of public key encryption relies on precisely this "small detail": there must be a way for the sender to obtain the receiver's public key through some other, secure channel. The definition of "channel" is actually very broad here, and denotes simply a means of transferring information.

To name but a few real world examples of what can be used to exchange keys: 


A trusted key server or repository, e.g. OpenPGP public key servers like sks-keyservers.net. 
A file downloaded from a website which is trusted to belong to the person whose public key you wish to obtain, and the connection to this website is secure and properly authenticated (i.e. using HTTPS). 
The receiver sends the public key on the same channel as the one it intends to receive the message, but the key's authenticity can be verified by a trusted external party such as a PKI (Public Key Infrastructure). 
A USB stick, disk, piece of paper or some other type of media handed over in person. 
A telephone call (!). 


Of course, one should nevertheless not forget that there is a myriad of attacks which can be performed on these channels too, such as using fake certificates issued by rogue CAs in order to bypass HTTPS or also flooding public keyservers with fake PGP keys. After all, a cryptographic protocol can only be as secure as the less secure of its parts.