Sweet Honey solves all your problems.
When using Sweet Honey while in a patch of grass, a horde is guaranteed to appear!
Unfortunately, Honey isn't very predominant throughout the game, but an unlimited supply being found on Combee (5% chance though).
But also, your friend Trevor will give you three jars of Honey, and there are there are four jars also avaliable in Route 5, Route 12, Route 18 and the Pokemon Village.
However, if you want a more available option you can use the move Sweet Scent, which also guarantees an appearance of a horde. A good plus to this is that a good amount of Pokemon can learn the move! Just check in the link to see what Pokemon can learn Sweet Scent. :)
Hope I helped. :)
Source and Experience