Have you tried Freecycle for a few roof tiles/slates. not so attractive but broken paving slabs would help until the roots are established. Really you just need something heavy, that the foxes can't move and something to keep the roots cool.

Thank you Urbanite I will try freecycle. I hadn't thought of that prefer slates , they look nice think I will wrap weed fabric around them so I have all winter to find slates the right size .

Have you tried local reclamation yards - you should be able to get roofing slates for about a pound each - or they may even let you have broken slates for free! As a last resort there's EBay. Lots of other options as well such as path edging tiles, quarry tiles, floor tiles etc. mellow down concrete paving slabs by painting them with yogurt to encourage moss and lichen.