1. You can go to
Beatmap Projects and create a thread for GD there for fast GDs. The downside is that there's not much guarantee in terms of map quality.
2. Simply use your social skills to meet more experienced mappers and then ask them for GDs. This way you won't have to worry if the diff ended up not satisfactory for you, but in the cost of more activity.
And yeah, as WitherMite said, while it does improve the odds of getting ranked, it's more to reduce work required in order for a rankable set, because even the set host themselves can make a full/rankable set w/out any GDs if they want to.