Best permanent injection for hips and butt?

This is difficult to answer without pictures. Sculptra can be used to enhance the butt. It may take a series of treatments, but can be combined with fat transfer and/or a butt implant to achieve desired results. With fat transfer, harvesting is important for successful fat transfer. I use gentle water-assisted harvesting to get the most viable fat to transfer. I combine with prp and sculptra to improve results. Results take 3-6 months. I recommend getting a formal evaluation with a body shaping expert. Best, Dr. Emer

Dear Amayamimi, The fat transfer will be the answer to your problem. In slim patients, it is hard to get enough fat for injection. Most of them have to gain weight to have enough fat. The amount of fat to be injected depends on how you want your hip to look. I would recommend you to visit a board-certified plastic surgeon to see how much fat you need so we can set up a goal for your weight gain. How long it will take to have enough fat is up to where the fat goes when you increase your weight. Fat accumulation can be either intraabdominal, under the skin, or both. Liposuction removes fat only under the skin. It will be ideal if all your fat goes under the skin, getting more fat for injection.

dear patient! Thanks for your question! It would be great to see some photos as well, and the best option for you would be to schedule several consults with Fat Transfer experts in the field, they should be Board Certified Plastic Surgeons as well!! (ASPS and ASAPS certifications and FACS as well)
I perfom at least two fat tranfers a week, and I have perfected the technique with very slim patients.  You should be ok with one procedure, and if you  havent achieved your desired look, you can then have more fat transferred in a second procedure.

Dear amayamimi,
it looks like you have a lot of questions. It is really hard to give you proper answer without any photos. I would recommend you to visit a board-certified plastic surgeon for an in-person examination and consultation. That is the only way you will get correct information. 
BBL can be used to enhance buttocks, but it requires that you have excess of fat somewhere else (abdomen, back, thighs, etc.). Again, you might be a candidate for a BBL but I can't tell without any photo or examination.
Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery 
Member, Am. Society of Plastic Surgery

Fat is the #1 choice for adding volume to hips and butt. There are a few products coming out for adding small amounts of volume (some products that act like fat transfer), but these are only very small volumes and are quite expensive. For those who are very slender and don't have enough fat, sometimes buttock implants are an option, but they require some additional recovery compared to fat transfer.