I really want a bbl, but I'm scared because my thighs are skinny. Can fat be taken from my flanks/stomach? (Photo)

You are a good candidate for BBL.  See the many patients who have skinny legs at the websites featured below 

Hello dear!
Thanks for the question and provided information as well.
Based on your pictures, you are a great candidate for liposuction with BBL, and I think you'll get very good results.
Liposuction allows your plastic surgeon to transform your entire shape. You can improve your abdomen, back and flanks, and of course your buttock, to give you a nice heart shape.
In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 33 or less.
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)

The most common donor sites for the BBL are the abdomen, waist and back. You  seem to have available fat in these parts as per your photos. Many patients present with thin thighs or with thighs that they don't want touched. You should consult with a Board Certified PS with extensive experience performing the BBL.