Its probably safer to get them from a shop as you can exchange faulty parts as soon as you think theres a fault,if you get any faulty parts that is, without waiting for the next fair and hoping that the same guy is there that time
When buying a case, think about where it is going to be placed.
I know people who spend a lot of money for fancy perspex sides with lights in and fancy fronts, only to have them stuck under their desk where they cant see them and they have to bend down to be able to see where to put the cd or floppy in
The main thing about the case is that its big enough to take whatever you want to put in it now and possibly at a later date and that if it comes with a PSU in already that it is rated high enough for what you need now and in the future