datwizz,
your assumption about buying tickets for that game is a correct one.
no point in buying them from anywhere else but the club website really.
you can download them to your phone and show them from there at the gates.
re: seats, check this out:
https://3ddigitalvenue.com/clients/sports/fcbarcelona
for atmosphere, the north end is anyone's best bet imo (north end is where it says 'rakuten' on the venue map).
as liverpool alluded, the 'ultras' ie the blokes w/ the drums and big flags and stuff, reside there, right behind the goal in the 1st tier.
north end is also where barca most often attack in the 2nd half of games and subsequently score most of their goals vs tired defences. **
sooo, if crazy is not what you're after but fun is i'd say get tickets from one of the corners near the north end. that's close enough to the noisiest bunch, but also comes w/ a safe enough distance away from them.
sections 137 - 139 in the 1st tier seem good for example. or 235 - 237 in the 2nd tier.
in the 1st tier you want to avoid the highest rows, tho. they're covered, which is good, if it's rainy out there, but you'll feel caged being there under the 2nd tier.
otoh, if you want the best view possible, nothing beats middle sections in the 2nd tier obv.
eta:
** well, at least they used to, nowadays ernie seems content to just have them sit back w/ the lead most of the time.