Bones of Burgos
You should get a copy of the 'Esplicación del Cuadro Histórico-Cronológico de los Movimientos y Principales Acciones de los Ejércitos Beligerantes en la Península durante la Guerra de España contra Bonaparte', por la Sección de Historia Militar written by Francisco Xavier de Cabanes. It's free on Google Books. A lot of the information used by Nafziger and Oman comes from this book I suspect.
The Estado Militar de España for 1808 is available from the Spanish National Library: http://hemerotecadigital.bne.es/issue.vm?id=0000711689&search=&lang=es
Regimental histories for the regular Spanish units are contained in the multi-volume: 'Historia organica de las armas de infanteria y caballeria españolas desde la creacion del ejercito permanente hasta el dia' by the Conde de Clonard. It was on the Hathi Trust website, although its pretty heavy going.
You may find this article by Spanish Napoleonic historian Luis Sorando Muzas useful as well: https://miniaturasmilitaresalfonscanovas.blogspot.com/2012/10/la-infanteria-de-linea-espanola-en-1808.html
If you have any questions about specific units please ask.
Have you tried the George Nafziger Collection at the U.S. Command and General Staff College Library? It is online and can be seen at:
There are thousands of orders of battles and a PDF index can downloaded at:
Do a search in the PDF and there will be a link next to the item in the index. Click on it and it will give you a pdf. For example the "Effective Forces Standing Spanish Army and Provincial Militias May 1808" lists all the regiments and their strength. It can be see at:
There might be a couple of options for you.
You could try and get a copy of Juan Sanudo's 'Base de datos, etc from the Spanish defence dept website. https://publicaciones.defensa.gob.es/base-de-datos-sobre-las-unidades-militares-en-la-guerra-de-la-independencia-espa-ola.html
I got my copy from Amazon for about the same price. The only problem is the program on the disk will only work on older laptops and certainly no Mac/Apple. This also gives you details of the variety of volunteer battalions raises.
You could also the the digital versions of the Spanish military calendar for 1807. This would give you an idea of the regimental commanders prior to 1808. I think I downloaded my version from the Spanish archive website. But I can email you my copy if you want. With Sanudo CD I have also downloaded the data for all nations involved in the Peninsula. So if you know the names of units you want info for I can send that also.
Thanks guys. I found info in Nafziger's OOBs.
Daniel I'm just starting my study so I have no specific requests at this time but I may have some in the future. Thanks