You could try approaching the managers of the chain toy stores near you, e.g. The Entertainer, Toys R Us, Argos, Early Learning Centre, Mothercare, and explaining what you are asking for and see if they have any clearance stock or any flexibility on price.
Can you approach a local branch of one of the bigger companies? There are usually "community budgets" for them to get free local advertising? It might not be ideal if the service users are very vulnerable of course to raise the profile.