Where to look for Vintage Cookbooks in Spanish?
I collect vintage books (birding books, though) and I speak spanish; I usually find good results in ebay.es 

I have checked for you on cocina books, and have found these:  http://www.ebay.es/sch/Otros-Libros-antiguos-/269/i.html?_nkw=cocina
the page is in spanish, so it conveniently filters books in spanish for you.   

If you are interested in one of the books http://www.ebay.es/itm/DR-A-VANDER-COCINA-MODERNA-/230916243325?pt=LH_DefaultDomain_186&amp;hash=item35c3ae3f7d
and want to proceed with a purchase, change the domain in the URL to ebay.com http://www.ebay.com/itm/DR-A-VANDER-COCINA-MODERNA-/230916243325?pt=LH_DefaultDomain_186&amp;hash=item35c3ae3f7d and voilà!

other interesting sources,
http://www.uniliber.com/resultados.php?pc=cocina&amp;orden=PA

http://www.iberlibro.com/libros/libros-cocina.shtml 

iberlibro.com is the spanish version of abebooks.com 
so you might get similar results searching there and filtering for cocina and a year range

http://www.abebooks.com/servlet/SearchResults?kn=cocina&amp;yrh=1950&amp;yrl=1930