[C.Rebecca]

If you think backlinking to .gov websites will give you good backlink credit, then I am afraid you have to look for another alternate. Most of the .gov/ .edu websites (if these websites are authority websites) have nofollow links.

Second thing, every backlink carries equal weightage (I mean extensions don�t affect backlinking credit) whether they are .gov backlink or twitter backlink.

But if you are really intent on it, you can use a tool called 'Keyword Country' for finding .edu and .gov backlinks. I use it to find quality sources for link-building and that too in a completely natural and legitimate way...