Unlike sound/instrument packs for GarageBand on the Mac, none exist for GarageBand on iOS.
This is why I switch to other third party all software iOS drum machine app/s.
I use this drum/beats machine software.
BeatHawk by UVItouch
Some styles come with app, others drum styles and sound styles are in-app purchases.
There are other drum machine apps, also, but this one offers the most flexibility and a huge library of sounds and drum/percussion sounds.
Look through the iOS App Store from other similar apps if you do not care for the one I linked to and use.
Every musician uses different tools.
Good Luck to You!