Cannot be obtained at any of the borders with Tanzania yet.
Here is the process: Apply at embassy in Dar at GPS: S 6.813581, E 39.290747. Need 2 passport photos, yellow fever certificate. Single entry costs USD45, valid for entry within 3 months allowing up to 30 days stay.
Process takes 5 working days. The visa fee must be paid at the Samora Av branch of NBC bank at GPS: S 6.820327, E 39.285366 (account details must be obtained from Embassy first, but currently are:
P.O. Box 9370 – DSM
Account #: 0121050004290
Take receipt from bank (one per person) back to embassy with application documents.
The passport needs to be left at the embassy for this period. If you are going to Zanzibar while waiting, take note, as the passport is needed for immigration there. The Moz embassy gave us a receipt stating they they were in possession of the passport together with a copy of the Tanzanian visa, and passport details page as proof for Zanzibar immigration. We didn't try to find out whether it sufficed, and bypassed immigration.