Income sources include
1. Home court ticket sales
2. Merchandise
3. Competition prizes
4. Sponsorship
5. Player transfer

Actually, you can check daily receipts of these items from Finance section

At low levels, as I remember, competition prizes and sponsorship are major income sources. But ticket sales will be more important as teams promote.

I believe that in very low level, you may still sign free agents in auction market. Besides, you will also periodically receive free young players from your academy players. Selling these free players is also good income source.