I am a home school mom (Technical Director) designing my set for our upcoming musical, Crazy for You. This is my third set I am designing and constructing...so I have little experience but have learned a lot. I need three small buildings to represent the street scene of Deadrock, AZ. I can figure out how to construct two buildings. But, the third (the Saloon) needs to be a double-sided or dual purpose building such that one side is the exterior of the saloon. And, when it is rotated becomes a lateral wall of the inside of the saloon (does that make sense??). The upstage and stage r walls of the inside of the saloon will be other walls that I need to roll in. I saw all of this on a photo on the internet. My questions...How do I construct walls that are about 8-10ft. high on a stage wagon without bracing? I have many flats in my stock right now. But, when I have used them as a fixed set, I have had the luxury of bracing them to death. Another question...when using a double-sided wall, it can't be a regular flat, right? Are there plans for a double sided flat? My flats are all soft. Another question...what are the best casters to buy? We had a small stage wagon in our last production where we just picked up casters from Home Depot. The wheels were always giving us trouble during production. They seemed to have a mind of their own! Another question...for the lateral walls not attached to the stage wagon, how hard is it to roll those onstage? I have read about a technique (can't think of the name right now) where the flat is affixed to a sort of skateboard looking thing (that has casters) and when ready to roll, the flat is tipped onto the skakeboard thing (wich is attached to the flat) that acts as a fulcum with wheels. Please forgive my description..its the best I can do without trying to dig through my Backstage Dictionary for the proper term.

I would love to attach pics of my inspiration photo to get feedback but don't know how to attach on this forum. If anyone is inclined to help me, I could send pics to them via email. Thanks sooooo much.

Have you considered using a scrim?  When you light one side it's the outside of the saloon and play is downstage of that. When you now light the people inside, the walls disappear and you see the inside.  This can be done by painting scrim/printing scrim as the  outside walls or can be done by projection of the outside walls.  Printed scrims are less expensive than people think.

Sounds like it's time to upgrade to "hollywood" or "studio" flats. These are flats framed with 1x3 ON EDGE. Unlike the typical "broadway" canvas flat which is framed with 1x3 FLAT and joined by cornerstones and keystones, Studio Flats are built with the 1x3 on edge and faced with 1/8" or 1/4" Luan (very thin, smooth plywood). This type of flat will be ~3" thick (rather than ~1" thick) and because it is covered in a hard material will be much more structurally sound, i.e. need less bracing.

As for the bracing: Consider building something like this:
http://www.carbon.utah.gov/north_spring/Castlegate/knightssaloon.jpg  
If it's a saloon, why not include the covered porch on the front? That would become the bracing and the whole structure would be self-supporting. If space is low, consider making the porch pretty shallow. You only need to make the whole thing about 2' deep total to get the look and support you need.

As for the side walls, why not make them part of the wagon? Consider using hinges to attach each side wall to the back edges of the center wall. When the unit is facing DS (with the porch on the DS side) the two side walls are "folded in" so they are parallel to the center wall. Rotate the unit 180 degrees (the porch is now US - and we are inside the saloon) and then "open" the unit up by swinging one side wall out SR and one out SL  to form a simple box set. The side walls will need casters on the DS edges to take the weight of the walls, but with the upstage edges hinged to the center wall, the wagon will take the weight on the upstage sides. Think of it like this: fold a piece of paper into thirds, like you would fold a letter. Now stand the piece of paper on edge and adjust the side flaps to create a little box set. Now you can fold the two side flaps so the whole thing is pretty much flat.

I hope this helps and good luck with your show.