I've got jars full of Sydney Showground, Amaroo MX & Dirt Track soil, a lump of Mt Druitt circuit bitumen*, a decent sized lump of Castlereagh Dragstrips hallowed start line, a lump of Liverpool Speedway bitumen and a rock picked up by Marty Tripes back wheel that hit me on the noggin (and hurt like buggery) at the Mammoth Mountain MX track in California. I've also got signs 'procured' from Amaroo, Warwick Farm, Oran Park, Liverpool Speedway and other tracks that are still in existence.
*Even though the Mt Druitt road race circuit was dug up in the sixties, you can still find bits of old track if you know where to look.