I think it is always more fun to find items - there is then a story behind it.
Buying items can be ok for some staple items that you otherwise wouldn't find, though a sharp DM ought to know what a player would really want bad enough and find some way to work it in through story than just through retail...
In the games I run, I tend to put in a lot of non-standard items that do both useful and strange things. One of my favorites was the indestructible dispelling staff. They found it holding up the ceiling in a circular room. It was quite an accomplishment getting it out of there, because as soon as they pulled it out, tons of ceiling would collapse. I even forget now how they managed it.
It really only did two things. It could dispell effects (more powerful than a dispell magic - like if you whacked someone turned to stone with it, it would turn them back - yes, that's how it worked, you had to 'whack' it on what you wanted dispelled) twice a day and it was completely indestructible, which makes for all sorts of interesting, creative uses.
It was of limited utility, but on certain occasions it allowed the party to surivive or escape where otherwise they would have been TPK.
Offhand, I can't think of any other items. But I do know it was fun for my players to find them - and they also really appreciated the custom items - much more interesting than another +X blah blah blah-oogle.