How to get hardwood

Hardwood is a scarce resource in Stardew Valley, and once you have foraged all the stumps for hardwood on your farm, getting more can be pretty tedious. There is only one area in the game where you are guaranteed to get a steady supply of hardwood, and that is in the Secret Woods.

Finding the secret woods

The secret woods is a hidden area directly west of Marine's farm, so in order to find it simply go to the farm and keep going left until you stumble upon a tree root. Log this and keep going left and you will enter the secret woods.
Because you need to clear a log to enter it, you need an Steel Axe or one of higher quality.

Logging hardwood in the secret woods

Upon entering the secret woods, there are three trees readily available for you to log. This will give you 8 hardwood. However, there are also two "secret" stumps you can find by going trough a secret passage. It's somewhat weird to explain exactly the spot, so use the images below to locate it.

This area has two more stumps to log, giving you 4 more hardwood, for a total of 12.
The stumps respawn every single day, so you can get 12 hardwood each this with this method.

Other sources of hardwood

 There are also a few other ways to get hardwood, such as finding them in crates and barrels in The Mine or the Skull Dungeon. This method of getting hardwood is very unreliable, but is a nice bonus when exploring in the mines or dungeons.