First, I think only in rare cases do Spain-based tour companies hire non-European Union citizens. Otherwise, they'd have to offer a work visa - which they're not likely to do.

Also, to be an official, licensed tour guide one has to undergo about 2 years of specialized training here in Spain. A friend of mine went through it and she told me it was VERY VERY difficult. She's completed her training, is licensed, but hasn't yet found a job in her field - after 2 years of trying.

Some of the big tour companies here in Spain would include: http://www.juliatours.es/ , http://www.trapsatur.com/ , but I'm sure there are more.

Saludos, MadridMan