I wear 38'' length and buy them at the Boot Barn on Center Street in Deer Park. Funny, Wrangler are the only ones that sell lengths over 36'' unless you have them special made. You can also find them ar Academy.
Try Denim Express. I buy mine there but mine are 29". Seriously they carry 38" in many waist sizes but most are an extra $2.00. They have free shiping on orders over $70.00. Of course there is no tax (yet) either. I buy there because they have the sizes in stock most of the time.