The best map currently available for Nicaragua is ITMB’s (International Travel Maps and Books) Nicaragua travel reference map (ISBN: 9781553413486).
Now in its forth edition this 1:755,000 scale map shows all the major urban areas and roads; the reference also highlights the roads that are only passable during dry weather, which is very important in Nicaragua. There is also a small insert showing a central plan of Managua and the legend is in both English and Spanish.
Although ITMB only has one retail outlet, they ship worldwide from their website: shop.itmp.ca, and their products are available from specialist map shops and major bookshops (like Borders) that have large travel sections.
If you are traveling to other countries in the region then ITMP also publishes a 1:1,100,000 scale map that covers the whole of Central America (ISBN: 1553410521), and the publisher, Reise Know-How, publishes a 1:650,000 scale map that covers Honduras, Nicaragua and El Salvador (ISBN: 3831771758).