You can buy the bags but I've always just made them from plastic and packing tape. I'm sure there are much better ways but this has always worked for me. Vacuum bag material and the black stuff that you use to stick the vacuum bag material down would work too. Remamber to make the bag a little larger than the size it has to inflate to to make sure the mould takes all the load and the bag is just a seal.

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!