I’m looking for information on the cheapest sim card and where I can purchase one in Spain for a mobile phone? eg a shop, web site etc. Thanks in advance for any advice.

Mobile phone shops and other outlets in every town.

I could easily have asked the same question, Rob. With over thirty years sales / marketing experience I cannot understand why every newspaper is filled with expensive whole page advertisements offering ‘better’ phone deals.
I went into a mobile phone outlet and changed (pre-paid). Fine, but before my tenner ran out I was offered a special deal via SMS. Buy a ?20 top-up and get ?30 calls. Great   ...  that is when I hit the proverbial brick wall.
I made a trip to the shop for the top-up. They had the cards but they hadn’t been activated. Could I call in tomorrow? I did - but they had sold out of the ?20 ones. Can I call in tomorrow? I did. Guess what. They STILL didn’t have any in stock. Can I call in tomorrow?
I asked him, was there a likelihood of them then being available. He said, ‘maybe.’  I asked the proprietor why the telecommunications industry freely advertise their competitors by failing to look after their customers. All I got was shoulder shrugging indifference. So I still have my money and no mobile phone. Great!

The answer to this:

‘I cannot understand why every newspaper is filled with expensive whole page advertisements offering ?better? phone deals. ?

is that they don’t in fact pay for the ads.

How can this be? you ask

Simple - the deal is that they share a percentage of the profit with the papers based on take up and sales.

Of course; why not   ...  welcome!