anti dustmite mattress encasing

[lsac0902]

Hi all! We found out after an allergy test that our son is allergic to dustmites. The doctor recommended to use a dustmite proof mattress/pillow encasing woven with microfine fibers. She gave me a brand but it's pretty expensive: close to $300 for a single size mattress! I'm just wondering if anybody has bought this kind of encasing at a cheaper price. I'm not looking for the waterproof kind, but those which are very tightly woven. Thanks!