Intra is within an organization while Inter is between them.

For example, Inter college football game is between 2 different colleges but intra college football game means a match between 2 teams from the same college!

As for source, just google or search the oxford dictionary online!

Here are two links to a list of words that begin with the prefix 'inter' and 'intra':

http://www.morewords.com/starts-with/inter/

http://www.morewords.com/starts-with/intra

thanks ^