How can I quickly get hardwood in Stardew Valley?

Solution 1:

There are 6 respawning stumps in the secret woods. They respawn daily for a total of 12 hardwood per day. If you go there every day you'll have enough in two weeks.

To get hardwood more quickly, you can grow mahogany trees. Mahogany seeds can be bought from the Island Merchant, or are rare drops from stumps and forest slimes. Hopefully you should see some while collecting hardwood in the secret woods. Once you have seeds, you can farm mahogany.

The Lumberjack perk is a level 10 foraging perk that causes hardwood to drop from regular trees. You can reset your perks at the Statue of Uncertainty in the sewers for 10,000g. If you have 10 foraging you can switch, deforest your farm, then switch back.

Solution 2:

It won't help you at this point, but if you choose to play on the Forest Farm map when you start a new game, there are 8 hardwood stumps on your farm that regenerate every day. Collecting hardwood from your own farm is far less time consuming than heading to the Secret Forest every day, and you can collect from both to more than double your daily quantity of hardwood (28 daily hardwood instead of 12). I've always had plenty of hardwood when playing on the Forest Farm.