Verified answer

The covers are there to reduce the evaporation of the pool water. This actually does a couple of things.

1/ Lessens your water loss daily.

2/ Helps retain heat when the pool is covered, reducing energy costs.

3/ Most importantly, it helps the actual enclosure. All that normally evaporated water goes somewhere. The structure. The indoor pools you saw with covers probably don't have a central dehumidification system, like a Dryotron. Those units start at over 40 grand and that's for a residential sized one, not commercial. The structure also has to have the proper venting for it, so an older indoor pool may not be able to host the ducting without an additional enormous expense.

Without somethimg to keep that evaporation in check, it can literally rain inside an indoor pool structure. Any glass, such as windows, skylights or doorways get ruined in no time as well. Steel beams in the roof supports not properly boxed in or otherwise sealed will also leech rust down where they meet walls.

You'll be able to order one for your particular application from any pool shop, either a flexible foam one or a bubble wrap one. You will also be able to order the reel system for it as well, if required.

Have you contacted a canvas tarp maker:

http://tarps.com/canvas.htm

Google and see if you can find one in your area. Make sure you tell them what you want to do so they make sure its mildew and water resistant.