How to choose participants for a study?
You will almost certainly have to recruit study participants yourself. As others have suggested, speak with your advisor to out whether there are any participant pools he's used in the past. If you find others that he hasn't used before it's a good idea to make sure he's OK with the pool before you dive in, so to speak.

Note that, in some fields, the pool of participants can be a significant limiting factor to the generalizability of the study results. Think very carefully about exclusionary criterion before settling on something. Note also that your IRB may require you to provide justification for exclusion/inclusion of certain populations (i.e., children, pregnant women, drug users). Try to make sure your population sample is as inclusive as possible.