Remember to get a Star Piece every day by going to Challenge Rock! And if you face Shellder or Cloyster in the wild, catch them, as they'll usually hold Pearl or Big Pearl. Paras, Parasect, Foongus, and Amoonguss also will usually hold a Tiny Mushroom. Also, I just say having your itemfinder/Dowsing Machine turned on at all times, it causes no effort on your part besides having to press a button from time to time as you go to the menu and heal your Pokemon with items or whatever, but just hotkey your itemfinder/Dowsing Machine and just press "Y" to turn it back on (or is it "X"? Easy enough to figure out, the PC should even be able to tell you.) This data comes from serebii.net.
Nugget: Nimbasa City, Chargestone Cave, Twist Mountain, Dragonspiral Tower (Winter Only), Moor of Icirrus (With Dowsing Machine), Challenger's Cave, Victory Road
Big Pearl: Route 13, Route 18 (With Dowsing Machine), Route 18 (With Dowsing Machine), Striaton City, Driftveil City
Balm Mushroom: Striaton City (With Dowsing Machine), Abundant Shrine
Big Mushroom: Route 1, Route 6 (With Dowsing Machine), Route 12 (With Dowsing Machine), Icirrus City (With Dowsing Machine) (Winter Only), Moor of Icirrus (With Dowsing Machine), Giant Chasm (With Dowsing Machine), Abundant Shrine (With Dowsing Machine), Lostlorn Forest
Big Nugget: Undella Town (This should give you the money you need)
Comet Shard: Giant Chasm
Pearl: Route 1, Route 4 (With Dowsing Machine), Route 13, Route 18 (With Dowsing Machine), Undella Bay (With Dowsing Machine)
Pearl String: Route 13 with Dowsing Machine
Star Piece: Chargestone Cave (With Dowsing Machine), Dragonspiral Tower, Giant Chasm, Giant Chasm
Stardust: Route 13, Route 13, Desert Resort, Desert Resort (With Dowsing Machine), Twist Mountain (With Dowsing Machine), Twist Mountain (With Dowsing Machine), Twist Mountain (With Dowsing Machine), Dragonspiral Tower, Dragonspiral Tower, Undella Bay (With Dowsing Machine)
iny Mushroom: Route 6 (With Dowsing Machine), Route 6 (With Dowsing Machine), Route 12 (With Dowsing Machine), Route 12 (With Dowsing Machine), Route 12 (With Dowsing Machine), Pinwheel Forest (With Dowsing Machine), Pinwheel Forest (With Dowsing Machine), Icirrus City (With Dowsing Machine) (Winter Only), Icirrus City (With Dowsing Machine) (Winter Only), Giant Chasm (With Dowsing Machine), Giant Chasm (With Dowsing Machine), Abundant Shrine (With Dowsing Machine), Abundant Shrine (With Dowsing Machine), Abundant Shrine (With Dowsing Machine)
Also note that each day there is a man on Route 13 who will give you an item. Sometimes it will be an item you can sell for a good amount.
Also, the Abyssal Ruins have items that sell for a lot.
You can buy these items in Black City, however, they cost more than they sell for.
Other Wild Pokemon also can hold these items, however, the chance of it is so low that they are not worth mentioning.
Remember to sell these items to the special item collectors. Also, I always sell any Honey or X-boosting items (which I have no need of for ingame). I also sell any Pretty Wings I get while hatching my Pokemon along the bridges (where I also get EV boosting Wings, which also sell for a good amount).
I included all this information to reduce the amount of like questions.
