I wouldn't trust any source other than the club website, the stadium itself or official Barça shops (of which there are many).I wouldn't buy from anywhere else as prices will be inflated, even at tourist offices, and definitely not from 'tourist weekends' with ticket included - it's cheaper to make your own travel and accommodation plans and buy a ticket separately.. The Camp Nou itself is plagued with touts on match days. You can purchase directly and print out the tickets at home. Tickets are on sale for this match at prices between 47€ & 154€ - quite a difference. Go here http://www.fcbarcelona.com/football/first-team/calendar
scroll down to the Real Sociedad match and click on 'start purchasing.' You will see where tickets are available. grey sections means reserved or sold out. There are plenty available right now and I doubt very much that this will be a sell-out. Use your mouse to hover over any particular section and you will see the price and the pitch view from that area. You will also see how many tickets are available for sale in that section. For a decent view at a "reasonable" price (everything is relative) I usually try and get a seat opposite the main stand and left of centre. There is a 'dividing line' where tickets on the right hand side are expensive and the left hand side only a couple of metres away half or less than half the price. This is also the only part of the ground other than the main stand with any form of cover. Many locals take pac-a-macs if it's going to rain (and if it's anything like I can see out my window right now then take two or three as one won't last 90 minutes!). Others take cushions to make things more comfy.
One very important thing to remember is that Spanish football is even more in thrall to TV than British football. Whereas Sky & BT have announced matches for the next few months, Gol TV only does so about ten days out. You will see on the Barça website that only the league opener this Sunday has a day and time confirmed. If you are going to purchase in advance you will need to be confident you can attend a day either side of the proposed date. In this case I would think it will go ahead on the date on the website but kick-off can vary. Not so much for a midweek game as for a weekend but even so...
I'd be fairly sure this will be played on the Wednesday with an 8.45 pm kick-off but sometimes it can be 10.00. Either you can take your chances now or wait till it´s confirmed by which time of course more tickets will have been sold, with less chance of getting four together (though I think you'll be okay with a midweek game not against a 'big name' opponent.
Whether buying online or at the stadium (many people buy at the stadium several hours before the game and also take in the museum which is well worth the trip if you can make it) you will need ID. A passport is fine. But remember to take your ID with you to the game and also the card you have paid with as often they will ask for proof of one or other or both.
There are no pay turnstiles. Entry is by ticket only. Place the front of the ticket against the scanner at the turnstile.
All seats and sections are clearly numbered so are easy to find. Toilets are plentiful and clean. There are food and drink outlets underneath every section. Alcohol is permitted if bought inside the ground. There is no smoking throughout though this is often flouted until a steward comes along.
Enjoy the trip and if you've any spare time PM me and let's see if we can meet up though as my wife's birthday is that week, I'm not 100% sure of our plans.