Nothing other than YOUR OWN fat is safe. That also has problems -- if often goes away and there is only a minimal result a year or two later. Many other things have been used, some have caused deaths and other such as industrial grade silicone have cause terrible problems. This is an operation that is not often advised.
The SAFE filler to the buttock is your fat. That is the BRAZILIAN BUTT LIFT. Underground fillers are dangerous and have caused multiple deaths in the U.S.