My 9 year old butt implants are uneven. Is a BBL to one cheek possible? (photo)

Based on the 1 posted photo YES! lipo of areas upto 1 liter of donor fat can be attained with transfer to 1 buttock. BUT, always a but, I require an ultrasound of the buttock to see if butt implants on larger side dislodged, rupture on smaller side, if in stable position. I offer this examination in my office or you can obtain an RX for ultrasound examination. I charge of ultrasound examine $500. If satisfied with the ultrasound examine then surgery can be offered fee $5,000. Best to virtual consult with....

The majority of plastic surgeons in the United States do not offer a gluteal implants. The main reason is that this operation has very high complication rates and is inherently problematic with issues like what you describe.
Your problem is not a lack of volume on one side it is that your implant has moved to an improper position most likely from not having been placed in the muscle but rather above the muscle and then sitting on the implant.
You don’t need a BBL you need an implant revision.
There is a handful of plastic surgeons in the United States who specialize exclusively on gluteal implants. These are probably the right doctors for your situation. You can also consider going back to the Doctor Who placed your implants nine years ago.
I don’t think fat transfer is going to correct this problem.
Best,
Mats Hagstrom MD