Question: I am looking for sheet music for a particular song I'd like to sing with my choir.
I tried googling and searching in some online shops but wasn't lucky.  

Is there some sort of specialized search engine, or catalog to search for choral sheet music?  

*--  

Sidenote:   
I am looking for a general answer to the question 'How to search for choir sheet music?' But if anyone happens to know the choir arrangement I'm looking for, I'd be happy of course :-)   
The song is "Wonderful dream" (aka. Holidays are coming, the song Coca-Cola is using in their Christmas Commercial).*

Answer: 
The most extensive site is <http://www.cpdl.org> . There are also many chorale arrangements on IMSLP (<http://www.imslp.org/>) but that site also has orchestral, solo, opera, pieces, etc. CPDL is specifically focused on choral music. What you will not find on either site is most music published in the past 50 years (or since 1923 for American users) since that music is in copyright. For that music you will probably have to purchase it online.
The sheet music with simple guitar chords and the vocal melody are in this link. <https://www.musicnotes.com/sheetmusic/mtd.asp?ppn=MN0119679>