These merchants feature mens jeans in 40 inch inseams:

Zappos.com

Zappos features Stetson and Cinch tall mens jeans in extra long 40 inch inseams.
Sale - 21 Days of Deals. Brand new deal everyday. Valid 12/1 - 12/21. Always Free Shipping & Free Return Shipping, so order several and keep the ones you like.

Tractor Supply

Tractor Supply Company features Wrangler 40 inch inseam tall mens jeans. Use the Size dropdown menu on the left of their page to find your size.