Well i think you have guessed, that finding liquor in Surabaya or Java in general can be a little bit difficult due to its majority Muslim inhabitants. And according to decision of Indonesia Trading Ministry : 6/M-Dag/Per/4/1/2015, beverages of up-to 5% alcohol may not be sold freely at mini market, this updated the previous decision of that sold freely.
Yet some stores have license to sell liquor. Once that the writer went was Ranch Market, located at Jalan Basuki Rahmat No. 16-18, Tegalsari, Kota SBY, Jawa Timur 60261, Indonesia. This supermarket has special room for liquors, and the selections are many. Also if you are healthy food enthusiast, Ranch Market also sells organic vegetables from local farmers. As the location is next to Tunjungan Plaza, you can visit Ranch Market when doing shopping or dining out.
Liquor can be found also at Bonet Supermarket located at Jl. Manyar Kertoarjo V No.62, Mojo, Gubeng, Kota SBY, Jawa Timur 60285, Indonesia.