You can get shots. Walk into any strip club and ask the girls there. I'm not kidding, I work in one, I just don't remember any of the specifics but a LOT of girls in there get the injection for their ass.
I don't think there are shots to make your ass bigger. Besides you should be happy the way you are. Trust me. I have a big butt and sometime it's a pain to find bottoms that can fit over it.