"There are readymade USB modules available from some vendors such as FTDI but again these are not easily available in most of the places."

This guy sells the FTDI chips. No MOQ, ships anywhere:

www.dontronics.com

"I wanted to know have you guys tried such ideas i.e. implementing USB by software bit banging"

Don't even go there. You'll be grey and bald before you get it working, and you'll almost certainly have to write a device driver if you plan to connect it to a PC.