I just got a vinyl cover/sleeve for my sons bed at walmart, it only cost about $4 for a twin size, it just slides over the entire mattress and zips at one end. It lasted a year or two before i noticed a few holes. I replaced it with a better sheet style waterproof pad. I dunno if thats the kinda thing youre looking for, but if it is, Iv found them in just about any store like walmart, kmart, meijer, and im sure you can find them in those types of places in different countries.
and the pretty basic ones seem to be fairly inexpensive. I dont remember the brand, it might even have just been store brand (i know the plastic one was) but the one I have no is soft top, and the sides are actual fabric so they wont tear so easily, I only paid $12 for it.