You can buy them on FC Barcelona's website
Tickets for Manchester City-Barça are still sold at a very high price. However there are tickets still on sale at FC Barcelona's website. On Barça's website and also in Les Rambles you can still get tickets, short before the Champions League round of 16 second-leg match.
Only 6 hours left for the match to be underway, there are still tickets available on FC Barcelona's website. Actually, there are still a total of 1546 tickets available between 59 on the stands, 675 on South goal, 202 on the wings and 610 in North goal. Tickets on the stands have a price of around 200 euros, on wings 150 euros and behind goals 100 euros.
On the other hand, there are also tickets on sale at Les Rambles. As you can see in this picture on @BMRisingTV twitter's account, the prices of the tickets are different from the ones sold on FC Barcelona's webpage. Actually, a lot of supporters have traveled without tickets, trying to reach the stands of Camp Nou during Wednesday.
Apparently, Camp Nou will be full, but short before the start of the match, there are still tickets available.