I had forgot that there were different meanings to the word. If the OP does mean a self-service place, s/he may be interested in the following:
Yin Tik Self-Service & Dry Cleaning Co 艷的洗衣店
Tel:2395 6332
83 Ki Lung St, Mong Kok
Otherwise, there are several places listed in the Yellow Pages, all of which were found by searching "laundry". The locations can be found by searching the YPMap:
Anmay Clean Centre 安美洗衣中心
Tel:2332 3291
Tung Shing Mans, Mong Kok
Bons Dry Cleaners Co 康兆乾洗公司
Tel:2782 2520
1009 Canton Rd, Mong Kok
Choice The Laundry 采絲洗衣店
Tel:2380 9252
102 Prince Edward Rd, Mong Kok
Chung Fung Kwan (listed in Chinese as 日昇洗衣店)
Tel:2989 1238
459 Shanghai St, Mong Kok
Clean Smart Laundry 智潔坊
Tel:2175 9930
Charming Gdn, Mong Kok
Lam Koon Wai Mark 全高速洗衣店
Tel:2787 6032
Yau Woo Bldg, Mong Kok
Mei Sun Dry Clean Co 美新乾洗公司
Tel:2332 7220
Winner Bldg, Mong Kok
New Fung Kui Laundry Co
Tel:2395 3729
Ka Lee Bldg, Mong Kok
Pak Ming Dry Clean Laundry Co 柏明乾洗洗衣公司
Tel:2398 8720
Poplar St, Mong Kok
Prince Edward Pk Lane Dry Clean & Laundry Co 太子柏麗洗衣公司
Tel:2380 3178
Playing Field Rd, Mong Kok
Shun Lee Laundry Co
Tel:2394 1561
1083 Canton Rd, Mong Kok
Tam Kwok Kan 御天使洗衣店
Tel:2994 0244
27A Shantung St, Mong Kok
Tse Wai Yu 卓越乾洗專門店
Tel:2396 6038
Kenwood Mans, Mong Kok
Wai Ka Dry Cleaner (listed in Chinese as 新天地洗衣店 -- again, a really different name...)
Tel:2392 0737
97 Ki Lung St, Mong Kok
Wang Hing Laundry Co 宏興洗衣公司
Tel:2381 3618
Lai Wah Ct, Mong Kok
Wang Sun Steam Laundry 宏日有限公司
Tel:2384 7711
Hung Cheung Bldg, Mong Kok