First, go to the Lionel Parts site, lionelsupport.com, and type in the model number (6-14579), then click on the hit that comes up:
Now look at the parts listing for "Traction Tire":
Your part number is either of these:
Go to Google and do a search for either, for instance "Lionel 610-6001-065".
You'll see that Trainz.com has them in stock.