Oh no, you don't buy them for money! People with shops make them for free. ^^ So just look around the shop and find something you'd like! I don't think you can make request threads since shops usually cover that well enough~
Kanto: Pikachu + Marowak/Arbok
Johto: Cyndaquil + Mareep Family/Scyther/Dunsparce
Hoenn: Treecko + Linoone, or Gardevoir, or Aggron... I see the issue here XP
Sinnoh: Turtwig + Mismagius/Froslass
Unova: Snivy + Bisharp
What about your favourite legendaries by generation?