on the gate, or on line. i collect mine at my brothers house............he lives in barca!!! off to see barca v mallorca in jan during my 10 day stay............CANT WAIT.
What you can't do is buy them on line and get them posted to England before you go. When we went, they were delivered to our hotel the day before we arrived.
bin a couple o times, best (and cheapest) option is to get up there the day before. one of the open top tourist buses stops right by the ticket office or go on the tube (les cortes, something like that) don't be put off by the unhelpful staff tellin you you can't get 2 (or more) together, just get them close unless it's v Real, sold out usually means 15,000 empty seats
If you're a little cheeky you can buy the cheapest tickets (the ones for the very top) and then work your way down the stadium when you're through the gates- it is pretty easy to get into a really good position like this- there's always empty seats. Just look out for the odd steward. I've done it a few times but my Barca living mate does it for every game he goes to.
You can buy them from machines (they might be the cash points) all over the city centre.