Well, not a website, but I think you would REALLY appreciate William Jurgen’s The Early Church Fathers, a 3-volume set, but you can buy volumes individually. The most valuable, IMHO, (because it covers the earliest pre-Nicene Fathers) is Volume 1, available for $12.71 (new) on Amazon.
It has a detailed Doctrinal Index. Pick a doctrine and look it up, and you will be directed to specific passages from the Early Fathers which treat the subject (quoted in the book) - even those writings contrary to doctrine which is recognized today (but was still controversial in early days). The books are invaluable and amazing! No Catholic apologist should be without at least Volume 1 (and owning the whole set is even better, and the doctrinal indexes of volumes 2 and 3 are cumulative of earlier volumes).