Handloader1 is correct. Use either .35 whelen cases or .30-06 brass and run them thru a 8mm-06 full length die cold....no heating required. Use a good lube and check your case lengths after resizing....they might grow somewhat.

I like a spray lube from Hornady or Lyman and I spray the cases from four sides intentionally trying to get a little in the case neck ID but not to spray into the case directly.