Viajes Mexico Istmo and Caribe (one block northeast of the zócalo behind the cathedral at the corner of Valdivieso and Independencia, tel. 951/516-2700 or 951/516-2919, fax 951/516-7475, 9 a.m.–8 p.m. Mon.–Sat.) is a good source for most travel services, especially air tickets.
Other agencies are more experienced at tours. Among these are Viajes Xochitlán (M. Bravo 210A, tel. 951/514-3271 or 951/514-3628, fax 951/514-2556, oaxaca [at] xochitlan-tours [dot] com [dot] mx, www.xochitlan-tours.com.mx , 9 a.m.–2 p.m. and 4–7 p.m. Mon.–Sat.). Go in comfort to all and any Oaxaca City  and Valley sites and villages, such as Monte Albán , Mitla , Teotitlán del Valle , Tlacolula , and much more. They specialize in individuals and groups, tourist and business services, and travel agent services, such as air and bus tickets and hotel bookings.
Another reliable, long-time travel and tour agent is Viajes Turísticos Mitla (at Hóstal Santa Rosa, Trujano 201, tel./fax 951/514-7800 or 951/514-7806, fax 951/514-3152), a block west of the zócalo’s southwest corner. They offer many tours, including guide and transportation, for small- or medium-sized groups. Their main office is at the Hotel Rivera del Ángel (F. J. Mina 518, tel. 951/514-3152), three blocks west, three blocks south of the zócalo’s southwest corner. They also provide very economical bus-only tourist transportation to a plethora of Valley of Oaxaca  sites, with multiple daily departures to Monte Albán . For more information and reservations, contact the bus departure ticket desk at the Hotel Rivera del Ángel (tel. 951/516-6175).
A Oaxaca regiment of private individual guides also offer tours. Among the most highly recommended is English-fluent Juan Montes Lara (Prol. de Eucaliptos 303, Colonia Reforma, tel./fax 951/513-0126, jmonteslara [at] yahoo [dot] com, $25/hour), backed up by his wife Karin Schutte. Besides cultural sensitivity and extensive local knowledge, Juan and Karin also provide comfortable transportation in a GMC Suburban wagon.
Moreover, satisfied customers give guide Sebastian Chino Peña rave reviews for his guided tours ($20/hour) anywhere you would like in the city  or Valley of Oaxaca . Contact him in town (cell 044-951/508-1220) or email him (sebastian_oaxaca [at] hotmail [dot] com).
More athletic travelers might enjoy the services of Zona Bici (Garcia Vigil 409C, tel. 951/516-0953, boccaletti [at] prodigy [dot] net [dot] mx), five blocks due north of the zócalo, which rents bikes and also conducts bike tours into the countryside.
For even more extensive backcountry adventure bicycling, hiking, and camping in the mountains on the pine-scented crest just north of the Valley of Oaxaca , contact Expediciones Sierra Norte (M. Bravo 210, tel./fax 951/514-8271, sierra_norte [at] infosel [dot] net [dot] mx, www.sierranorte.org.mx/home ).
Also, you might inquire for a guide recommendation at the state tourist information office (Murguia 206, tel./fax 951/516-0123, 951/502-1200, info [at] aoaxaca [dot] com, www.oaxaca.travel , 8 a.m.–8 p.m. daily).