Psuedomonas (PA) is one of the more common bugs that CF patients contract, and you are correct in that it can be hard/impossible to completely eliminate.
As for where, unfortunately PA is common everywhere, including in soil & standing water. It has been found in various household products at a contaminant in things from shampoo to mouth wash. The best you can do is be consistent on hand-washing, following the guidelines for keeping away from others with CF to prevent exposure and then live life.
Ultimately, I think most with CF will culture PA at some point, and I don't know if you can prevent, but signs show that you can delay with some effort.