I'm not familiar with any of the crossovers that you are looking for, but have you tried the crossover page for Supernatural on FF.net? It shows stores for all of the ones you are looking for except Elfen Lied.

Here is the link: http://www.fanfiction.net/crossovers/Supernatural/2237/

Edited 2012-03-04 07:27 pm (UTC)

A Loaded God Complex (http://spn-summergen.livejournal.com/36099.html) by gwendolyngrace
Summary: The demon-worshipping cult that John has been tracking chooses Sam’s Middle School as their target. But John doesn’t know that there’s already another investigator on the case. Medium crossover (but pre-series, so not necessary to know anything about Medium).