Large distributions and shops
Many shops hire extra personnel at set periods of the year to help with sales, at the till, for deliveries, for handling, for stocktaking, for commissioning, for packing… Focus your research according to the seasons (the sales period, Christmas…)
Think about seaside resorts that are crawling with seasonal work! Contact them from February/March.
Most big retailers recruit online, on their website, for summer jobs or via tempting agencies (p.13). For all other boutiques go and contact the managers directly with your CV and a cover letter.
Ice-creams, drinks, doughnuts… This summer job is difficult: you must be in great shape as you have to walk across beaches under the sun and pulling the weight of your cart or your icebox. Remuneration is very often based your number of sales. Be careful! Send your CV directly to the Ice Cream Parlours from Spring.