At the time the land was transferred to you the paperwork should reflect the fair market value at the time you inherited the property.
If it does not and you have no paperwork on the value of the inherited property you can try a real estate firm. They should be able to advise on the value of other property in the same area as the land you sold.
Document how you came up with your cost basis for the timber and keep it for your records.
Your basis in property you inherit from a decedent is generally one of the following.
The FMV of the property at the date of the individual's death.
The FMV on the alternate valuation date if the personal representative for the estate chooses to use alternate valuation.
If you received money from the sale of timber, you'll probably get a Form 1099-S. Some people will get a 1099-MISC, which is (technically) the wrong reporting form per the IRS.
Regardless of which form you received, timber income is reported in the investment section. Just follow these instructions; don't enter the sale in the 1099-S or 1099-MISC section or you'll go down the wrong path.
For your cost basis, most people will enter a 0, but you could enter what you originally paid for the seedlings or saplings. On the More Info About This Sale screen, you'll want to select Everything Else, located at the bottom of the screen.
Tip: TurboTax doesn't support IRS Form T (Timber) but the good news is that you probably don't need this form. It's only needed "...when a sale or deemed sale under sections 631(a), 631(b), or other exchange has occurred during the tax year" to quote the IRS. See the IRS instructions for Form T.