Big and tall cotton work shirts for tropical climate?

My boyfriend is going to be spending 3 weeks in the country of Honduras on an archaeological dig, during the month of July. The dress code specifies long-sleeved shirts that button down the front, and the shirts must be 100% cotton. I also need shirts like this in 2X large, extra-tall. I am having trouble finding shirts made of this description that are made with durability in mind--all I can find are flimsy dress shirts. This needs to be a work shirt. Any sources for online purchase, or national stores that would carry this sort of thing? Thanks.