You can find a Ralts on Routes 203 & 204 only by using the PokeRadar
You get Eevee from the Trophy Garden by asking the Owner what pokemon are in there.(Save before talking to him and reset if you don't get Eevee.
Riolu is an egg and is given to you after you complete or when you reach Iron Mountain.
Togepi can also be found on Route 203 by using the PokeRadar.
Please note that the PokeRadar can only be obtain by going inside Pal Park for the first time after beating the Elite Four.
Something you can do is use the GTS and try to find the pokemon you desire