Carrefours would probably be the best place, or the car accessory shop in the car park of Carrefours. But make sure it is just a booster seat you need, as I know people have been fined for not having one suitable for the age of the child!! Or alternatively there is a hire place in San Luis that hires all sorts of equipment for holidays.

Hi
I go two at Carrefours and they were quite cheap. Alternatively if you look at the webpage for Kideasy they hire baby and nursery equipment. They are British and very reliable. if you want they will meet you at the airport with the seats and meet you there to collect again when your grandchildren are going home. Their site is www.kideasynurseryhire.com.

If you only want to hire them. Charity 4 Charities have them to hire out for a small donation to their children´s charities. They also have pushchairs, cots, baths, and high chairs.
There all in good condition.
You can tel. 965724905. Roy & Danniela.
Or go to the Bull Flanagan, Quesada, on a Tuesday. They do an Auction for Charity.

Thanks everyone. Bought them in Carrefour. They were, however, 21€ each!

Lot of money. Anyone else who´s interested. Charity 4 Charities hire all baby eq, wheel chairs etc. All they ask for is a small donation to charity.
Aprox €2 per day, depending on the item required.