Wellbutrin (bupropion.) Switch while you're breastfeeding. It is the gold standard, and well-studied: an almost undetectable amount is present in your milk, and is not at all detectable in the blood of the infants when tested one hour after breastfeeding.
I white-knuckled it through seven months of pregnancy, and then when I just couldn't bear it any more, my OB happily (and expressing the wish I had told her I was struggling sooner) put me on Wellbutrin for the last three months of pregnancy - she regularly prescribes it to women who smoke and are pregnant as a smoking-cessation aid - the risks of smoking are far, far greater than the risks of buproprion (and many other antidepressants.)
I then went on to breastfeed for two full years, ultimately switching back to Zoloft at one year. My son is a happy, verbal, active, growing, sharp, funny, healthy boy.
I totally understand not wanting to medicate your child unnecessarily - but I suggest (from experience, and with much gentleness) that some of your anxiety about the "risk" is a product of your depression, and treating the depression will help you to more accurately assess this, and other, risks.
Here's some solid research, if you are the type that is helped by research (I am):
http://www.kellymom.com/health/meds/antidepressants-hale1...
You can also google and come up with lots of studies, all basically asserting that all retrospective studies (a woman and her doctor decide to medicate, and then record blood levels in mom and baby and take breast milk samples) suggest that there is little to no impact of bupropion on the baby and that a very few mothers have trouble with a dip in milk supply. Since no one can do a double-blind placebo study on pregnant/nursing mothers and any medication, no one is ever going to be able to say it is definitively without any risk. But all sources will tell you that the risks of untreated depression and anxiety are real and quantifiable.
I will also say, especially since depression can make emotionally connecting with your baby challenging, if you are otherwise enjoying breastfeeding, I would encourage you to find a medication you are comfortable with and continue breastfeeding, for the emotional connection if nothing else.
That said, if you still feel that you are not comfortable with continuing breastfeeding while treating your depression, formula is a safe and available option that we can all be grateful for. Your baby will be fine either way - as long as you get the help that you need. Untreated depression and anxiety are a far, far greater risk to your baby than any medication you could take.
Talk to your doctor. Talk to your pediatrician. Talk to your OB. Although it can take a long time (six to eight weeks) to get an appointment to see a psychiatrist, many practices have psychiatric nurse practicioners that you can get in to see within a week or so, and can share their expertise and experience with nursing mothers. Whatever you decide to do about breastfeeding, you are being the best mom you can be by taking care of yourself and treating your depression.
You are doing a great job, be gentle with yourself. Best of luck.