Maps — Oaxaca City, Oaxaca
The Oaxaca state tourist office (Secretaría de Turismo) publishes and distributes excellent glossy maps of the city and valley (published in English and Spanish) for free. These maps are small, they're not difficult to fold up, and they're easily tucked away or pulled out for referencing when you're out and about for the day. Many hotels give them away at the front desk. If your hotel doesn't have them, there are two places conveniently located in the Centro where you can get them: the tourist office at Independencia 607 (across the street from the Alameda de León) or the tourist booth next to the Iglesia de Santo Domingo (intersection of Constitución & Alcalá).
A large glossy detailed map with the city on one side and the state on the other side can be bought from some of the newspaper and magazine vendors that congregate at the intersection of Trujano and Flores Magón off the southwest corner of the Zócalo for Mex$50. The map is too cumbersome to tuck away and pull out when you're out for the day, but it is more detailed than the free maps available from the tourist office.
The Instituto Nacional de Estadística y Geografía (INEGI) is a Méxican federal agency dedicated to collecting, organizing, and publishing demographic, geographical, and economic information on México. Because they're a government agency, they have a lot of maps and census data you can consult for free. They also sell topographical maps covering most of México.