it depends on what kinds of effects you're looking for 
idk about ibis paint but i know that if u have a cloud account for medibang you get acces to all of their free assets and materials so maybe you can try there? idk also suggest whipping up some of your on on nedibang if you have a solid idea of what you want like simple sparkle brushes or patters are pretty easy to make so long as you can work out the shapes!
like i haven't got access to my bookmarks rn but just searching through artstreet materials page can turn up a ton of free downloadable brushes and materials. some (if not most at this point) are paid but theyre usually very cheap so dont hesitate to browse those pages as well. theyre practically a dollar at the least for some and packs just a bit more than that 
hope it helps~
