Hi Tessa,
There is more about mattress encasements (that surround the mattress on all sides) that can protect against dust mites and bedbugs in post #2 here and the sources of information that it links to that may be helpful 
As you mentioned … a fitted sheet style mattress protector wouldn’t be suitable for bed bugs although it would certainly help to some degree with dust mites because it would help prevent skin flakes and moisture from entering the mattress which is their main source of food and liquid.
Phoenix