I cannot provide the precise answer you are looking for, regarding the name of an agency. The easiest and cheapest way to get to Niger, is with Point Afrique, a charter airline that flies to Niamey, from Marsailles or Paris. (see their web site www.point-afrique.com)
I flew with them in 2001, and will do so again late this year. They used to fly to Agadez, but presently the airport is not servicable for international flights. In Agadez, I organised a camel trek, but there are lots of Agencies, including Dune Voyages, which can organise 4WD expeditions. I don't think it is too difficult to organise once there -particularly now that tourist numbers have declined since Agadez airport was closed to international traffic. The trips involving 4WD are not cheap.
There is quite a good coach service between Niamey to Agadez, taking about 12 hours, but I don't think that it runs every day.
[This message has been edited by Alistair (edited 08 July 2003).]