Where can I buy sawn logs for all the different plots in Hearthfire?

Solution 1:

The wood needed in the Hearthfire DLC is distinct from the wood you can obtain from any chopping block. You are looking for Sawn Logs. These can be purchased from several log merchants in the game:

• Aeri - Anga's Mill in the Pale
• Bolund - The wood mill in Falkreath
• Gilfre - Mixwater Mill in Eastmarch
• Grosta - Heartwood Mill in the Rift
• Hert - Half-Moon Mill in Falkreath
• Hod - Wood mill in Riverwood
• Kharag gro-Shurkul - Solitude Sawmill in Haafingar
• Thonnir - Wood mill in Morthal

Some of these merchants will also allow you to chop lumber from free if you complete enough radiant quests to raise their disposition. Note that if you saw your own logs, the merchant must be nearby for you to actually receive the lumber.

Solution 2:

There are only a few NPCs that will sell you Sawn Logs (these are different from Firewood). The UESP has a handy list:

It sounds like you may not be talking to Hod, but one of his employees (Faendal, perhaps?). If you can't find him, consider one of the other NPCs.