https://www.postgresql.org/support/professional_support/
Debian has a list of consultants, many of them are Debian developers:
https://www.debian.org/consultants/
In addition there are a bunch of general and specialised open source consultancies, some examples:
http://www.credativ.us/ https://www.igalia.com/ http://collabora.com/ http://codethink.co.uk/ http://catalyst.co.nz/ http://www.sysmocom.de/ https://www.savoirfairelinux.com/