hitch hicking is possible accapt in the border area. watch out for truck drivers on petrol stations or food stalls along the way.
there are no thrugh Bus service from Tunis to Niger. from Tunis take a Bus to Nefta. here you can find Taxis with a licence to cross the border by In Hazoua to El Oued or better Touggourt. with the Bus to Ghardaia and further on to Tam. public transport from Tam to Agadez means, standing on a pick up, or ridding on the top of the load from an Truck.
its not verry wise to look out for an Algerian Visa in Tunis. Make it at home. so you have no problems and you saveing time.
Niger Visa are aviable in Tam.
time off processing: 24 our
best wishes for an great adventure