An XL Easy Walk Harness or SENSE-ible Harness should fit, with room to grow. You should be able to find at least one of the two at a store like PetSmart, but if not, you can buy them online through many vendors. (I bought my Mastiffs' first harness online, but found the second at PetSmart.)

Easy Walk Harness:
http://www.petexpertise.com/dog-collars-dog-harnesses/no-pull-dog-harnesses/easy-walk-no-pull-harness.html
http://www.puplife.com/products/premier-easy-walk-harness

SENSE-Ible Harness:
http://www.softouchconcepts.com/products/sense_ible_harness.php

Just keep in mind that while these harness are excellent tools and great safety precautions, they are not a replacement for actual training!