138 NI Act

For Sending the Notice :

If it was the firm I will consider as 

Partnership firm: To managing partner name and write the address which is in the firm registration Certificate

Example:

To

Mr.xyz

Managing partner,

Address: ______

______________

In case if it was the Proprietor firm then send to the proprietor office or residential address

Send it to the signatory of the cheque directly.  Under the NI Act only the signatory of the cheque can be impleaded as the accused,  with exception of a registered company. Although you can CC to the other members of the firm that is your choice.