hmm, well i really think a simplier veil would look best since the dress is pretty elaborate already, but if you (or whoever, if this isn't for you) have your heart set on a mantilla (and i can understand, i really wanted one but just decided it was too much for my dress/my height) your best bet is probably to find someone who can make one for you. it doesn't seem to be a popular choice these days and even some bridal shops looked at me like i had two head when i asked about them.
veilshop.com does have them available, and i think they do custom stuff too:
same deal at glamgal.com, some available, some things they can custom make: