Although they are not too difficult to find, here's some
Hidden Hollow Locations. Hope this helps! I'm missing Route 2, 12, and Abundant Shrine, though.
Edit: Added Route 2 & Abundant Shrine. Feeling lazy.
Route 2
- Enter route from Accumula Town. It's above a female Veteran and to the left of this Lass.
Route 3 (post-game)
- This one is shown on Serebii's site. The southern end of this route has tall grass.
Route 5
- Enter route from Nimbasa. Head northeast. Bianca showed you this one.
Route 6
- Enter route from Driftveil. Go directly above the female Pokemon Breeder.
Route 7
- Enter route from Mistralton. Take the stilts to go straight to the hidden hollow.
Route 9
- Enter route from Opelucid. Head west, and past the shopping mall building, you'll find the hidden hollow tucked away in a corner.
Route 13
- Enter route from Lacunosa. Head south, either taking the Cut shortcut or going through the grass (has female Pokemon Ranger). The hidden hollow is then to the left.
Route 18 (post-game)
- Enter route via surfing from Route 17. Make your way to the house (where a female Ace Trainer heals your Pokemon). Go west and down the stairs, you'll see the hidden hollow.
Route 22
- Enter route from Shizui's town (8th gym). Take an upper route and find Terrakion. Go east through grass and get to the female Ace Trainer. Take the stilts with the S patterns. Take another set of stilts leading you to the hidden hollow.
Route 23
- Enter route from Victory Road gates. Head southeast and take the bridge heading south. After that first bridge, go right. From the fighting-type trainer, go down. Continue on this path then and you will find the hidden hollow at the dead end.
Sangi Ranch
- Enter ranch. Head northward into the grassier area (the area you fought a Plasma grunt). Surf right.
Giant Chasm
- Enter route from Giant Chasm cave. Go straight up, and you'll find the hidden hollow in the first grass patch you touch.
Abundant Shrine
- Enter area. It's northeast, near a Youngster.
Lostlorn Forest
- Enter forest. Go to the northernmost wall. The hidden hollow is above the male Pokemon Ranger.
Pinwheel Forest (post-game)
- Enter route from Skyarrow Bridge. Head south and enter the gap in the fence on the right. Make your way to the river and surf left.