Did you happen to save the placenta? Not a certain cure, but often a big help if you make it into pills and take them as needed to lessen the depression.
If you feel like you are physically drained Eleuthero (sometimes called Siberian) Ginseng is safe to take while nursing, and while not an antidepressant per se, it gives a nice boost to energy levels to make everyday tasks and just getting out of bed feel less daunting, which can be one of the hardest parts of depression. I found it to be a life saver about 3 months after I had my baby and just hit the wall. I took 1 dropperful in the morning and one in the afternoon for 3 weeks and felt relief for months afterwards.
Like others here have said isolation is the enemy, being alone with a baby day in and day out will drive pretty much anyone over the edge. You need to be around other women who have been through this, women who can make you laugh even when you are sad.
Make sure your diet is rich in omega 3's, fish, fish, fish. Those babies steal our reserves to build their own brains like crazy, so extra is definitely needed to maintain our own mental health. If you choose to start taking fish oil, start slowly and ramp up over a few weeks because a sudden spike in concentrated fish oils can just change the nature of the depression if the brain is suddenly flooded, and create serotonin syndrome.
If it comes right down to it, don't fear antidepressant medications, they can be very helpful, life-saving even. Some are safe to take while breast feeding, but if your doctor prescribes one that isn't you can spend a few weeks pumping as much as possible to extend what you have for your child before starting treatment. That said, breastfeeding itself is supposedly a natural antidepressant so definitely talk with a knowledgeable doctor before discontinuing it in search of a cure, and perhaps try a trial of talk therapy for a few months before considering meds.