The little plastic thing on my sd memory card for my camera has dropped off and it has made my card read only.

Is there a computer or camera shop in Granada where I can buy a new one?

First of all you can fix it with some sellotape.
Just wind  a piece of sellotape around the card tightly.  With this solution you can make the card get stuck inside the camera so it is best to buy a new one.

The best and biggest computer and  shop in Granada is Media Markt but it is right on the edge of the city and would cost you at least 20 euros in taxis to get there and back.

The best place to go is the Corte Ingles department store right in the centre.
They have a camera and computer section on the 3rd floor.
It is opposite the informatica department.

Here is a link to the corte ingles page.
https://granadamap.com/corte_ingles/

Look at the print out a map link.