How to get a record related to a lookup value and another attribute

I have the following scenario:

1. A Payment (custom entity) has a lookup to a Cost Code (custom entity).
2. A Cost Code record can be related to 1:N General Ledger (custom entity) records.
3. General Ledger has an Item attribute.
4. Payment has an Item attribute.

When creating the payment I need to get the associated General Ledger record based on the Cost Code selected and the Item value on the payment.

I'm suspecting the only way to do this is with custom code.  I really need to expand my knowledge and learn how to move outside of the box. 

I have spent the last year learning Dynamics CRM and have built a relatively complex social services case tracking system for our local government, which includes welfare payments and accounting entities (with MUCH help from this community).  I have squeezed every ounce of juice out of the lemon that I know exists -- there may be more I don't know -- but I'm sure I've hit the ceiling.

Where do I go now?  I want to expand my capabilities and be able to do what I mention above.  I'm a self-taught person, so don't shy from the challenge.

Thanks for any guidance!