To get a cost basis do some due diligence the best you can, otherwise you have to enter a $0 cost basis.
Take a realtor that specializes in land sales to see if you can get some historical data.
Many agricultural colleges keep data on FMV of land.
As long as you use good sound logic, and have some evidence of how the value was arrived at, IRS can be somewhat reasonable in this area. Just document what you arrived at, and how you arrived at it.