Actually it's quite easy to make your own clear display case at a fraction of the cost compared to buying pre-made. Not to mention by making it yourself you can create any size.
Now your probably thinking how in the world do I make a display case.....
First, buy some clear acrylic sheets (plexi-glass, acrylite) <----these are just the name brands, places like Menards and Home Depot sell acrylic sheets in various sizes and thicknesses. If they don't have what you require, check the yellow pages for local plastic suppliers, though keep in mind they may be more costly the Home Depot. Another tip, bigger sheets are usually more cost effective then buying a small sheet.
Second, cutting your acrylic. It is possible to cut plexi on a table saw, though it's recommened you use the special plastic/plexi blades they sell. It will provide a cleaner cut(less cracking) and won't melt the acrylic in the places you are cutting compared to a regular wood blade. Or if you can use a laser cutter. This provides close to perfect edges, and you can engrave the acrylic with a name/label/design if wanted. Yes, their expensive so look at plastic suppliers/professional laser cutting services, some plastic suppliers use them to cut the acrylic. For a display case you just need rectangular pieces cut, they can do that easily if you ask just provide the measurements.
Third, if you've gotten your acrylic cut now it's time to assemble. I personally use Liquid Nails Clear, it doesn't mar the surface as it doesn't actually melt the acrylic. So even if you make a mistake just grab some paint thinner and rub of the stray glue. Wait a few hours so the glue dries and guess what your DONE!!