Where can I find information about extending a Chinese tourist visa in Xi'an, Shaanxi?

The Xi'an Public Security Bureau 西安市公安局 has a page which lists a number of locations with entry and exit functions:

• Economic and Technological Development Zone Entry and Exit Administration Hall Address: Weiyang District Fengcheng Road 16 (Fengcheng Road police station on the east side). Tel 86259667
• Beilin public security entry and exit accreditation hall Address: South Second Ring Road slope Ding word Changan Road police station west. Tel: 029-86772722
• Metro public security entry and exit hall Address: New City West Road after the door 51 (Provincial Maternal and Child Health Hospital next door). Tel: 029-86752435
• Wild Goose Prison Public Security Entrance Office Address: Yantai District Yan Zhan Road 250 (National Exhibition Center North Gate North). Tel: 029-89315021
• Lianhu Public Security Entry and Exit Office Address: No. 129, Xiguan Street, Lianhu District (next door to the Fifth People's Hospital of Xi'an). Tel: 029-84354942
• Baqiao Public Security Bureau entry and exit accreditation hall Address: Baqiao District spinning all the way on the 23rd. Tel: 029-86758183
• Weiyang Public Security Bureau Entry and Exit Administration Hall Address: Noah District Xuanwu Road 50 (Weiyang Branch Daming Palace police station next door). Tel: 029-89379723
• High-tech Zone Public Security Bureau Exit-Entry Office Address: High-tech Zone High-tech Management Committee (City Gate) Block B on the west side of the first floor. Tel: 029-65690800
• Chang'an Public Security Bureau Exit and Entry Office Address: Chang'an District Changan Road South (Houjiawan integrated market). Tel: 029-89250855
• Lintong Public Security Bureau Exit and Entry Office Address: Lintong District, South Street, on the 5th Street. Tel: 029-83825150
• Yan Liang Public Security Bureau Exit and Entry Office Address: Yanliang District Yan'an Street on the 9th. Inquiries: 029-86879139
• Huxian Public Security Bureau Entry and Exit Administration Hall Address: Huxian County Road No. 12. Tel: 029-84817558
• Lantian County Public Security Bureau Exit and Entry Office Address: No. 173, Changping Road, Lantian County. Tel: 029-82726474
• Gaoling County Public Security Bureau entry and exit accreditation hall Address: Gaoling County Grass Street No. 53. Tel: 029-86911485
• Zhouzhi County Public Security Bureau entry and exit accreditation hall Address: Zhouzhi County Erqu Road, No. 32 Road. Tel: 029-87154235
• City Exit and Entry Reception Hall Address: Science and Technology Road No. 2. Tel: 029-87275934

I imagine you are interested in the relative ease of the extension process in Xi'an. With a quick search I found mixed results:

• An older blog post from 2010 claims it's relatively easy (at least compared to Beijing) and gives some instructions (see also the comments there for more or less recent changes to the process)
• some posts in the LP forum as of 2014 instead make it look rather cumbersome, with a lot of time needed, and this travel report from the same year indicates similar.
• Another report from 2013 suggests similarly that it takes on the order of ten days to process the visa extension.

Obviously, you will need to go to one of the Exit & Entry offices. An online search in Chinese gives the address of the Visa Office of the Exit & Entry Administration Division, Xi'an Municipal Public Security Bureau: 2 Keji Road (at the intersection with Taibai South Road), Yanta District, Xi'an. It is in the transportation police building, near the Taibai South Road (Taibai Nan Lu) subway station and the Xixie 7th Road (Xixie 7 Lu) bus station.

I am rather sure that other exit and entry offices only issues passports and border permits to Chinese citizens and don't deal with foreign citizens. It is suggested that you directly go to the office at Keji Road; that one for sure will deal with visa issues.