Most cases of this nature are too complicated and take too long to be handled on a pro bono basis. If you cannot afford an attorney the court will appoint one for you once the case is filed.
Your fiance needs a great lawyer, not necessarily a free one. I would start calling local attorneys to see if anyone could help, if not pro bono, at a reduced fee. Save/borrow whatever you can to fight this case.
I know you believe he is innocent, but he is in jail because the State thinks he is guilty. Don't be naive about the process. There are innocent people charged, arrested, and even convicted every day across Texas. You need experienced counsel to assist your fiance. For a capital case that is never cheap, but it is always necessary.
Your fiance should request a court appointed lawyer. If he is indigent the court will provide an attorney to represent him.
Finally, do NOT discuss this case with your fiance, either over the phone, or in writing. The jail can record his conversations and use that information against him. Only attorney-client communications are privileged.
Disclaimer- This is not legal advice. You are not my client, and I am not your lawyer. You need to hire an attorney to assist in this matter.
A capital murder case involves hundreds if not thousands of hours of work for an attorney. Most are not in a position to just drop their practice and take one of these cases on for free. However, there are a few organizations that pay their attorneys and allow them to take on capital murder cases for free. One of these organizations is the Texas Defender Service (who I have linked to below). They cannot take every case so you'll have to go through the intake process.
If they don't take the case, then the court is obligated to provide competent counsel (and co-counsel) in a capital case. Still, I think you would be well-advised to consider Mr. Guest's advice. This is not the kind of case where you want to cut corners.
Google the American Bar Association guidelines for a capital murder defense. You will see two things. First, just how exhaustive and exhausting this work is. Second, how the state must pay for a highly qualified attorney, investigator, experts, (And More). I have handled over 20 cases charged as capital murder cases. By the time you are done, the costis close to 1/2 a million dollars. If someone does the case pro bono, be very careful and ask two questions.
1. Are they able to meet the ABA Guidelines, pro bono.
2. What are they getting in exchange. (Few people do 1/2 a million dollars in legal work pay for experts, investigators etc. for no reason except their desire to do good deeds.
I have never heard of any attorney doing a capital case pro bono and I would be suspicious of such an arrangement if I did. Your fiance needs a qualified attorney. If he cannot afford one, the court will appoint an attorney to represent him.