I understand there may be a Niger embassy in Tripoli, but not in Tunis. In Tripoli it's at Hay AI Andalus, Rue Ahmed Bin Abdoul Malik. The telephone number is 478 16 38, mobile 091 214 6570. I found these details on the internet so I can't vouch for them.
The Rough Guide to West Africa says that French embassies don't issue visas for Niger but Ivoirian embassies sometimes do - but that is not going to be much help.
Let me know how you get on. We're planning to go Libya to Niger at the end of the year and it would be good not to have to get the Niger visas until Tripoli. At present we're thinking we'll get them in either Brussels or Paris.