The only permanent filler I know of is fat. Normally, in my experience, about 60-70% of the fat takes and then becomes permanent.
Thank you for posting your question. There are really no safe permanent fillers on the market today. There are doctors out there who could tell you Silicone is an option, but I would strongly urge you to avoid this. At this time, the safest solution for enhancing your buttocks would be with fat transfer. This is a very predictable procedure which can provide great results with low risks.