Who should I ask references from for my computer science master's application? I am graduating around this October-December with my BSc in computer science and I want to apply for MSc degree abroad. However, I need around 2-3 referees to write the reference letter. Typically in Finland you don't have much contact with the professors personally. Only one that really knows me academically is my bachelor's thesis adviser who is most likely willing to write me a letter because my thesis received a really good grade.

But the other two references seem harder because I don't know the professors well really. For example would it be okay to ask someone like the department head if I show him my transcripts and CV even though he doesn't really know me personally?

In general, who should I ask the other two references from in my school if I only know one professor well? Technically, I will take a few courses before graduating still so I might have a chance to interact more with the professors but in Finland the student-professor interaction is not really strong usually.
Your best course of action is to send an e-mail or approach professors who have taught you and in whose class you achieved great grades. If they agree to, you can mention that you would be willing to discuss your interests/experience in a mini-interview, and provide any documentation (CV, transcripts, description of program, etc.) that they might need to write a strong LOR.