Well, the closet thing you can do to "buying" a Water Stone in ORAS is trading Blue Shards.
Here are the methods to obtain evolutionary stones:
Water Stone:
Route 124 (trade for Blue Shard with Treasure Hunter), Mauville City (Inverse Battle Stop), Secret Super Training (The Water Stone Cup Begins!, The Battle for the Best!), Route 106 (rematch with Fisherman Elliot), Secret Base ("Pick up stones" with Secret Pal)
Fire Stone:
Fiery Path Route 124 (trade for Red Shard with Treasure Hunter), Mauville City (Inverse Battle Stop), Secret Super Training (The Fire Stone Cup Begins!, The Battle for the Best!), Route 114 (rematch with Kindler Bernie), Secret Base ("Pick up stones" with Secret Pal)
Sun Stone:
Mossdeep Space Center Pickup, Mauville City (Inverse Battle Stop), Secret Super Training (Drag Down Hydreigon!), Route 115 (rematch with Expert Timothy), Secret Base ("Pick up stones" with Secret Pal)
Held by wild Solrock (5% chance)
Shiny Stone:
Route 121 Mauville City (Inverse Battle Stop), Secret Super Training (An Opening of Lighting-Quick Attacks!), Mt. Pyre (rematch with Fairy Tale Girl Momo), Secret Base ("Pick up stones" with Secret Pal)
Thunder Stone:
New Mauville Route 124 (trade for Yellow Shard with Treasure Hunter), Mauville City (Inverse Battle Stop), Secret Super Training (Follow Those Fleeing Goals!, The Battle for the Best!), Route 117 (rematch with Triathlete Dylan), Route 118 (rematch with Guitarist Dalton), Secret Base ("Pick up stones" with Secret Pal)
Dusk Stone:
Mauville City (Inverse Battle Stop), Secret Super Training (Watch Out! That's One Tricky Second Half!), Mt. Pyre (rematch with Hex Maniac Valerie), Secret Base ("Pick up stones" with Secret Pal)
Leaf Stone:
Route 119 Route 124 (trade for Green Shard with Treasure Hunter), Mauville City (Inverse Battle Stop), Secret Super Training (The Leaf Stone Cup Begins!, The Battle for the Best!), Route 118 (rematch with Aroma Lady Rose), Secret Base ("Pick up stones" with Secret Pal)
Dawn Stone:
Victory Road Mauville City (Inverse Battle Stop), Secret Super Training (Those Long Shots Are No Long Shot!), Sea Mauville (rematch with Mysterious Sisters Scall & Ion), Secret Base ("Pick up stones" with Secret Pal)
Moon Stone:
Meteor Falls Pickup, Mauville City (Inverse Battle Stop), Secret Super Training (Drag Down Hydreigon!), Jagged Pass (rematch with Expert Shelby), Secret Base ("Pick up stones" with Secret Pal)
Held by wild Clefairy and wild LunatoneAS (5% chance each)
Source: bulbapedia pages for each stone :P
So yeah, no, sorry.
Hope this helps! :)