Ikea Erfjord mattress - mattress cover shrunk to the size of a bib! Help!?

I bough an Ikea Erfjord latex mattress in the as-is section and decided to get the mattress cover (which was filthy) washed. Now, I know the tag said there is only one way to wash it, which was with upholstery cleaner, but the drycleaner said he would wash it cold and hang dry. Even at that, the mattress cover still came back the size of a dish towel. What's worse, it would seem I can't even buy this mattress cover separately. Ikea was saying they'd take it back with a receipt if they had an equivalent mattress cover in their as-is section, but I went today and they didn't have any. I don't know what to do!