I would guess the guy that built them probably went to Menards, or maybe another big box store. Maybe one of the local yards. I know a couple of the feed and tractor stores around here sell some of the barn building supplies too. Yer just siding a barn/garage, you can use whatever siding is available to you locally. Check around at your local contractor stores, if you have feed and tractor stores around you. If you already have an architect or contractor working with you on this, ask them what they might be able to source. What the most sturdy design is... that's depending on the design. You can get great materials and a poorly designed building that isn't sturdy. It looks like you want some standard and oversized roll up doors. Make sure the structure is designed to support that. If you don't have some gutters planned, you might want to plan some in. If there is supposed to be loft storage or upper story inside there, make sure you build it up to code.

There is a bulge in the wall, below the picture of the words right side of toilet. If it was the toilet, it would not have caused damage there, and that bulge, appears to be from moisture. In any case, buy a new wax ring for the toilet, and remove the toilet and check to see if it is leaking. Its not a difficult job, and can change the wax ring, and have it back down, in 10 minutes, once you check to make sure its dry. All that white markings on the floor are also water damage, so one way or another the toilet and flooring need to be removed anyway.

Is there any such thing as a waterproof vanity, if not what is the next best thing, and does anyone have any recommendations on where to buy? Thanks!

I went on the Owens Corning website and they have a "find a contractor" area. I've gotten some references from neighbors also. I don't recognize any of the contractors listed on the Owens site, but it does say: "Owens Corning Roofing Platinum Preferred Contractors are the top tier of our exclusive network and meet strict standards for professionalism, reliability and unparalleled craftsmanship. Only they can offer our best roofing system warranty." Do you think it makes sense to use their list?