Levelling the Garden

I want to level my garden, as it is uneven with a slight dip at the far end. The soil underneath is clay, and when the gardener laid the turf last year, they failed to properly level the ground before doing so, resulting in a poor job.

Garden is healthy otherwise, recently having Worm crests and mushrooms every now and then.

I have heard that i need Sports sand (without compost or mix) for drainage and level it.

Any suggestions where to buy it from or what else can be used?

Thank you.