Chop large stumps with a Copper Axe or better
Chop large logs with a Steel Axe or better
Break mahogany trees for hardwood
Get hardwood from the Secret Woods stumps and logs
Find hardwood in the Quarry Mine
Get hardwood from the Forest Farm daily stump spawns
Buy hardwood from Robin on Fridays if available
Get hardwood from the Traveling Cart when sold
Recycle certain items through the Woodskip or other sources that yield hardwood-related rewards