Honestly, unless you get very fortunate, anyone at Home Depot will more then likely look at you with a blank stare when you ask them anything about a walk in or humidor building in general. If you have a lumber yard such as 84 lumber, these people tend to know lumber much better then your average Home Depot employee. I am not saying the HD is not a great store, but when you expect your employees to know where everything is in the store, you limit their ability to know something very well.
Anyhow, I bought some 4 X 8 sheets of 1/8" spanish cedar through 84 lumber for 165.00 a sheet. At the time I was going to do a walk in, but my wife and I are not sure if we will be in the house much longer and are thinking of moving to North Carolina, so, until we decide, the cedar waits for my use. Selling it to you would be ridiculous since it would cost an arm and a leg to ship.
Call around to your local lumber stores and ask them if they cary spanish cedar, if the person on the other end of the phone gives an answer that sounds intelligent, such as " yes we do, or, no not a large market in that, what are you using it for" then you talk some more, if they tell you they have cedar and don't understand the difference, RUN!!!
Most won't sell 4X8' sheets, they usually come cut down to like 2X4' sheets, or smaller, just depends on the lumber yard and who their suppliers are.
CC's comments on the green drywall are spot on, and the Water based glues are water based until dry, and then dry impermiable to water and are not broken down by water. The reason to go with these glues relates to vapors and the release of vapors. Just as any pungent or heavy smell can permiate food and other organics, same goes with your cigars, and how disgusting would it be to Fire up your favorite corona only to have it taste like varnish, or glue?!
There are many professional grade glues, a few are made by Elmers, I believe it's called Bond or something like that? Great wood glue that dries nice an oderless and actually contains sawdust to aide in the joint strength.