the theatre light covers are called gels - though if they have a 'real' name I've never heard it in 5years of building, fitting up and operating stage sets! They're a bit like overhead projector film, but generally much more colourful. They do of course make a massive bunch of frosted, semi opaque and diffusion type gels.
They're also designed to melt when they get hot, rather than burn which is good, so DON'T PUT YOUR FINGERS IN IT

it hurts. It stays as a sheet, just on the verge of liquid - its really bizarre
I can easily get hold of some if anyone asks nicely I can get the lighting man at the theatre to put some extra diffusion gel onto the order next time they run out, or cut some off the big sheet if they've got enough in the gel rack if they're not ordering anytime soon... but I have no idea how much it costs?