The two spy items are awesome, but I love the Cloak and Dagger the most. Spies used to be nearly useless on large games on maps like 2fort since the corridors into the base are always being pyro spy checked and you're always running out of cloak at the ends anyways. With the Cloak and Dagger you don't have to risk running…
I'm not really sure, I found two items within the first 2 hours, then 1 during the next two, and then just got the Cloak and Dagger during the hour after that. For me it seems to be an average of an item an hour.