Hostelworld is a web-based company that specializes in finding and booking inexpensive accommodation for tourists on a budget. It's span is world-wide - their Prague page is an example.
I checked out their search engine for Arusha this morning, and got some results I recognized, though their price bracket - less than $50 - does not seem to be well represented close to the city. However the page is easy to use, and responsive, and does appear to have very wide coverage.
Since Adia's place fits in the right cost category, we'll probably be listed there as soon as I have read though the fine print and determined that we can meet their basic administrative requirements.