Where can you intentionally download computer virus online for testing purpose?

You may want to look at the EICAR antivirus test file.

You can create this file by opening a text editor and pasting the following into it:

X5O!P%@AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*

The above text, in the words of the above site, is actually x86 machine language, and "... is a legitimate DOS program, and produces sensible results when run (it prints the message 'EICAR-STANDARD-ANTIVIRUS-TEST-FILE!')."

There are various research sites that keep malware samples available for research. I found a list of sites that provide downloadable samples here: http://zeltser.com/combating-malicious-software/malware-sample-sources.html