Open Question: How to find customer service jobs?

I realise this sounds like a stupid question, but - I want to find a basic customer service job for the summer, preferably somewhere like Tesco, Argos, Asda, Boots etc. But none of these big chains seem to actually advertise customer service roles online - their websites just have management or head office roles! So how can I find out about these jobs? Do I need to go in and ask if they're hiring? Or do chains like these do all their 'low level' recruitment through employment agencies? Thanks!