There are several organic fertilizers, but for seedlings, I would use the half strength seaweed fertilizer after the second set of true leaves. You run a risk of burning the tender roots if you use full strength.
It actually is not recommended to fertilize before you transplant. You will stimulate rapid, new growth, which will sap the root strength of your new plants and may cause transplant shock. It is recommended to harden them off for a week or so by exposing them gradually to the outside temperatures. Water them well before transplanting and plant in the evening so the sun and heat do not draw moisture from the plants and increase the chance of transplant shock.
Here is a link on transplanting to help:
Please reply back if you need more information.