There are several things you can do:
- Set a good Marshal to educate commanders; he will invite high command non-landed Greek commanders now and then. He wil also boost the stats of the existing commanders.
- Get the forged bloodline that attracts heroes. They will also tend to have a high command skill + be of your own culture/religion.
- Use the court tutor to educate promising martial youths.
- Invite a strategist from China and set him as a Marshal; He will boost the stats of your existing commanders.
- Do a character search on high martial skill; usually, if the ruler has high enough prestige there will be several willing to join your court (sometimes for a fee). You do not even have to land them. Just assign as commanders.
You can't have unlanded commanders as Byzantium.