Hi summerfruit, we got our dust sheet from the DIY section at the supermarket in the wekly shop.
B&Q or homebase will have them otherwise.
The one thing I found though is that having a plastic sheet on your bed(well under the fitted sheeet but YKWIM) makes the bed really uncomfortable.
I covered our mattress in a dustsheet when I changed the sheets just before DS4 was born and that night when I wentto bed I had to take it off again, it was a right PITA!!!
I just replaced the plastic sheet with a few towels instead. As it was I didn't need either as the MW broke my waters when I was at hospital.