I agree with Babelfish - see a perinatal psychiatrist if at all possible. I've found mine so great at putting my mind at ease.
Mine suggested two websites for finding info on pregnancy and breastfeeding and different drugs.
I also like pubmed.gov. They do have some very limited studies on breastfeeding while on seroquel (quietiapine).
Good luck! Try not to be too hard on yourself. I'm sure whatever you decide will be the right choice for you and your baby.