If you can take prozac alone, it is less likely you have bipolar because those types of drugs can spring a bipolar person into mania if they are not prescribed with a mood stabilizer like lamotrigine or lithium. Once you get medicaid it would be worth going to see a psychiatrist and get tested.
For cheap drugs check out Walmart. I think, although I could be wrong, that generic prozac is on their list of discounted drugs. Under $10 if I remember correctly.
Yes as regards insurance perhaps you would be eligible for Medicaid. If you went to a walk in clinic you could be eligible for emergency Medicaid which could transition into standard Medicaid. There are also clinics with a sliding scale fee and other insurance options as well. There is an independent living center in each county of each state and they could advise you of insurance options. Other agencies could be of help in this regard as well.
From what I've heard antidepressants can make bipolar mania worse although sometimes during the depressive part of the bipolar cycle doctors will prescribe an antidepressant like Prozac.
Just because you have mood swings doesn't necessarily mean you have bipolar. Bipolar is really difficult even for mental health professionals to diagnose. Symptoms can be similar to borderline personality disorder and also some subtypes of schizophrenia. Also there can be comorbidity between different conditions. Apparently 20% of people with bipolar also have borderline personality disorder.
Anyway if you responded well to Prozac last time then I guess you could try and get some again. Ask some friends about social services in your area. A social worker might be able to help you out.
Getting prescription drugs in NZ is fairly cheap. I think I paid $3 for 90 fluoxetine pills.
Good luck and hope everything works out for you!