Cassava is such a popular crop in that area I would have assumed it was relatively easy to find saplings.

You do understand that Cassava is propagated from stem cuttings as the tubers do not produce buds. Stem cuttings should only be taken from plants which are free from disease, are at least 10 months old and have borne tubers. The cuttings should be taken from hardened stems leaving at least 30 cm (11.8 in) of stem intact in the ground. The stem can be severed using a sharp knife, secateurs or saw and each cutting should have 1-2 nodes and be approximately 20 cm (7.9 in) long.

So, have you approached local farms or farmer centers to obtain cuttings?

Based on your comment that you are in Dar es Salaam then you should contact the following for virus free stock
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Edward Kanju
Edward Kanju is a cassava breeder at IITA based at Dar es Salaam, Tanzania since 2002. His research is focused on developing new cassava varieties that combine dual resistance to Cassava Mosaic Disease (CMD) and Cassava Brown Streak Disease (CBSD) for the eastern African region.
Dr Kanju obtained his Ph.D. in Plant Breeding from the University of Free State, South Africa. He was by then employed by the Ministry of Agriculture, Food Security and Cooperatives, Tanzania. His research background includes cassava and sweet potato improvement especially developing new varieties that combine resistance to major biotic and abiotic stresses along with end-user-preferred characteristics.
He has been supervising several projects in East and Southern Africa in collaboration with NARS in Burundi, DR Congo, Kenya, Mozambique, Rwanda, Tanzania and Uganda.
For further information:
Address: Plot No. 25, Mwenge Coca-Cola Road, Mikocheni, P.O Box 34441, Dar es Salaam, Tanzania
Telephone: +255 22 2700092
Mobile: + 255 754 897182
Email: E.Kanju@cgiar.org
Nationality: Tanzanian