I always start by emailing the club directly. (due to the fact some websited dont translate) Usually works. 24 games abroad(non Dons related) and only one club did not reply Basel(stuck up c@nts) but got a ticket on the night. Borussia Dortmund replied but were sold out and they apologised. So i used Footballticketnet.com and paid double which was about 20 quid(glad i did as it was 8-4). Turned up at my hotel in Dortmund and my ticket was waiting for me. Got a reply from everyone else, alot of the teams held a ticket and only wanted a scan of photo id and i picked up on the day and showed them the id. Either that or they sent me a link to where to buy. I just booked my next trip (Fortuna Dusseldorf) a few days ago. Went for a shower after i booked and emailed and i had a reply by the time i was out.
Its a doddle usually.
To note my old boys been(Shakhtar in the CL) and he loved it so much his diddy team is Valencia and he pretty much watches ever game they have on sky
But i would advise a) go on their website and see if you can buy if not find a contact and email them