I don't know where you will get the cheapest, but I have always used:

Lucky Lucky Motorcycle
No. 413Eo, Preah Monivong (St. 93), 12258 Phnom Penh
Tel 023 220 988
Tel & Fax 023 212 788
Mobile Phone 010 998 099
Mobile Phone 012 279 990
Mobile Phone 099 808 788

or

Sophiya Travel and Tours
Phnom Penh Head office:
#216B, St.63 (Trasak Pa-em), Boeung Keng Kang 1, Chamkar Mon, Phnom Penh, Cambodia
Tel.: (855-23) 222 455, Fax: (855-23) 996 322, H/P: (855-12) 962 902
E-mail: info [at] sophiyatours [dot] com

For all my visa extensions.

A 1 year business Visa is normally acquired by the company you are working for/sponsored by and takes three days provided you have arrived on a 30 day business visa.