Tickets are allocated to the away team, so these would have to be bought from their website.
There also 2 official ticket exchange websites where season ticket holders who cannot attend are allowed to sell their tickets, but do not know if an International match would be included in a season ticket. Stub Hub and Viagogo. May be worth a look.
I also found this site -
http://www.easysportstickets.com/sports/football/celtic-vs-fc-barcelona-tickets-23-11-2016?gclid=CP--wPyP6c4CFQWVGwod4tcDhg
but do not know if they are trustworthy and ticket is nearly £300.
Hopefully someone into football will be along later.