Hi Jonaz,
As we have an 18 month old and another baby due imminently we've had to do this kind of shop for our apartment. You'll be pleased to know there are a few branches of Toys r us in the area - which also have the babies r us section as in the UK. Same kind of stuff as in this country - travel cots, booster seats, saftey gates and everything else you might need. Sorry I can't remember what sort of prices we paid as it was last summer. Carrefour supermarkets also stock a lot of that kind of stuff in their baby sections (these are like large Tescos if you've not been before).
Assuming you know the area a little - the easiest branches of Toys r us to find are one directly next door to Ikea in Murcia and one on the main shopping complex as you come down the A30 into Cartegena (also a large Carrefour / Leroy Merlin there)
I also remember reading some small print on the easy jet website that says you are entitled to take a travel cot free of charge on a flight so you may find you can take a purchased one over with you. I'd double check that though as that was a while back.
Hope you get what you're looking for.
Caroline