I would adopt from an orphanage or adoption centre...  But in Canada, at least, we never hear about abandoned babies...  I don't know if that's because it's hidden or not, but I would imagine that due to how available adoption services are, and how in-demand babies are for infertile couples, that "abandonment" is very rare indeed.

And no, I wouldn't adopt an abandoned baby..  I would probably take it to a shelter.  It would also, of course, depend on where/how I found this baby, too.