Condolences to you my friend. Incredible story, the universe does weird things like that sometimes...
I can't help with Barca home end tickets but you might even have a chance with getting a ticket in the away Utd end if you have/get a Utd membership (£20). Barca away tickets will be sold to season ticket holders and members in priority of highest number of previous European away matches attended...however most people seem to think that this game will go down to 0 credits, so you would have a chance of getting a ticket without attending any previous matches. If I was you (assuming you are a Utd fan), I would try this route first as being in the Utd away end would make the experience even more special IMO.
If that route fails, then I would look at trying to get in the home end. Given the size of the Nou Camp, I'm sure you would be able to find one relatively easily although you may pay a premium on the ticket. I went to a Barca La Liga home game a few years ago - we bought tickets from Viagogo but I had a quick look on there now and can't see the Utd game so maybe they can't sell on there for European competitions. I'm sure there are other unofficial websites but I can't personally recommend any. If you buy from one of those, then definitely use credit card rather than debit card to pay so you have some additional buyer protection from your bank.