Like Andy, I have some Ikea glass cabinets, in two sizes, plus two glass door bookcases that I use for my model displays.
However, at some point in the future, I'll need a case for a 1/48th scale Lancaster diorama, the base of which will be around 24 x 28 inches.
I found a couple of UK web sites where they will cut clear acrylic to the required size, and send by courier, and this looks like it will work out at around £20 to £25 for the required pieces to make a cover case to fit the diorama base, plus the clear epoxy adhesive with which to but-join the sections.
I also want to add extra shelves to some of my cabinets, and again, the clear acrylic works out a lot cheaper than the equivalent glass cut to size, which surprised me. The shelves work out at around £5 each, at a size of approximately 34 inches x 11 inches, whereas bevel-edge glass would be more than £15, nearer £20 !
If you don't have room for a full height cabinet, try searching (probably on e bay), for ex-shop display cases, the type that are 'counter height', used for jewelry, perfume and similar small objects, as they can often be found at reasonable prices.
Edited :- re-checked prices.