I recently got a very good deal on a new set of 5 * 5.00 6 ply tyres and tubes from
Budget Pilot Supplies: The Pilot Shop for Cheaper Pilot Supplies
Had a problem with slow leaks so I had to replace the tubes too. Aircraft tubes are surprisingly expensive. Car tubes are probably < $20, plane tubes > $100.
From memory, a pair of tyres and tubes (including freight) from Qld to Melb) cost $440, which was a heaps better than the ~$600 I was quoted by someone else.
They had them in stock and they arrived two days later.