I can recommend DSC Showcases. He even delivers them personally.
You can enter your required size on the website and get an immediate quote. After spending £300 to £1000 on a kit and 2 to 3 years building it, how much is too expensive to protect your creation? My DSC case is quite large, so made of 3mm glass and very is heavy. It takes two of us with glass suction handles to get the case over the model, which we do on the floor, then lift by the plinth onto the table. Not something you move very often.
Plastic is a lot lighter. Premiere Showcases do them in acrylic and offer a flat-pack option that, as I understand it, doesn't include the sheet. I just don't see the point of that.
I have considered aquarium suppliers but never really investigated that option. My next model will get another DSC cover, they're that good.
I may try and talk him into 2mm glass this time though!