Usually any type of guide finds you first, either through dreams or meditation. Spells don't really help with that, I believe.
Try meditation and if you see a dragon during meditation and then keep dreaming about the same dragon, and/or keep seeing it in your meditations, that is probably your dragon guide.
But...your guide may not necessarily be what you want it to be. I wanted/thought mine was a wolf, but it's totally different than that.
It might be different for Dragon guides, but I do not study them, but I do know about spirit guides in general.