Mountain Lion has several sources for scanner support. This Apple article lists scanners and printers that they have:
To update the support for those scanners, they have links for driver updates from these vendors HP, Ricoh, Canon, Epson,Brother,Fuji/Xerox which if updated after June 25, 2012 support Mountain Lion.
These other sources have scanners:
Open source support from: http://www.ellert.se/twain-sane/ together with http://www.sane-project.org/sane-mfgs.html
Some HP ones have a trick you can do as documented by this thread:
http://www.hamrick.com/ Vuescan has support for these scanners: http://www.hamrick.com/vuescan/supported-scanners.html
Image Capture support http://macmanus.nl/2013/11/28/use-unsupported-scanner-in-mavericks/ (also lists Mountain Lion)
http://www.silverfast.com/show/mountain-lion/en.html has Mountain Lion support for numerous scanners.