It might be worth signing up to a credible exchange with large BTC volume (Binance and Bittrex are good options).
You will get IBAN/SEPA/whatever details required in your exchange account and you can deposit the fiat to the exchange.
Then you can open a trade for 50 BTC and probably have it filled in a day (a few hours most likely).
Disadvantage: You might have to undergo more intensive KYC processes though as you would be a high volume trader. This could take a few days or weeks (you would be able to deposit and trade, but not withdraw till the KYC is completed).

LBC is convenient, but as you have already experienced - the local sellers would most likely not hold that many coins. You would probably have to make many deposits to different users (they are also restricted by limits) over a few days to get 50 BTC. I have personally also found that LBC can often be expensive.

If I were you, I would try the exchange route - unless large cash deposits attract attention in Spain. You are sure to also save alot as you will get your BTC and pay less than 0.5% trade fee in most cases