goldman sachs internships during the year

I am new to all of this, but am wondering if Goldman Sachs and some of the other top ibanks (e.g. Morgan Stanley, JP Morgan, Citi?) offer internships during the year for students not currently enrolled in school (I just finished a non-econ related master's). I know consulting firms are fairly strict about this and do not and require that one be enrolled in school at the time, but I've heard ibanks are more flexible. If so, any advice for how to go about this? I would be interested in interning in the winter/spring. Also, there are tons of divisions within GS--which are the best/most rigorous/most prestigious? Thanks!