I'd go to route 240 or 255.
Garitina
Not really that rare... in Pokemon Diamond you can find Sunkerns on Route 204
all the Pokemon on route 210 in Pokemon diamond and pearl are ponyta, geodude and if your really lucky you can find onix.
Slather honey on the trees, or get really lucky in tall grass.
In the desert. (No really, simple logic in the water!)
You can find Pidey in a Pokemon Outbreak in Pokemon Diamond
Well unfortunately you can not catch it in diamond. The Pokemon is slowbro if you did not know and if you really want one you can trade it over from a leaf green or fire red.
While finding wild pokemon, if you are really lucky, you will eventually find one. However, there is a less than 1% chance to find one.
It appears randomly in the wild, but it is really hard to find and it is very rare.
== == you really cant tell because they're different in pearl/diamond so sorry.com?????????????????????????????
YOu find Reilley at Iron Island. You go really deep.
You can't really find them all in diamond, but you can either trade, or migrate from others games. Hope that answers your question