Where Can I Legally Buy Juvederm to Inject my Own Skin?

You can't legally buy these products in the US because you don't have a license to practice medicine, and any product you import from another country can't legally be injected since it hasn't been approved by the FDA. 

I cannot caution you enough against using DIY injectables. First, the product you purchase online is unlikely to be FDA-approved, which is necessary to ensure the safety and efficacy of the filler. It is also impossible for someone who is not a board-certified injector to know which dosage and needle placement will produce the desired result. In addition, incorrect injection technique can lead to potentially serious, and even deadly consequences, including nerve damage, facial paralysis, and stroke. Schedule your treatment with an experienced, board-certified injector if you are serious about dermal filler treatments. The risks are simply not worth the money you might think you would save with the DIY approach.

I would agree with the majority of my colleagues on here and advise against doing this at home. 
Treatments such as botox and fillers should only be given/ prescribed by qualified practitioners.. There are a number of practitioners out there who carry this out incorrectly and they have been trained so to consider doing this without any formal training would be very dangerous.
As a plastic surgeon, I have treated many patients who have had fillers injected into the wrong area/ developed excessive lumpiness etc and these are problems that can completely distort the anatomy and leave you with a much larger bill as reconstructive surgery following complications is always more complicated and generally much more expensive.
If the majority of the clinics throughout the US/UK and Europe and probably in most other countries do not sell this 'over the counter' - this must tell you that there is a reason for this. Buying products that are known to require a licence but are suddenly available - to me is a red flag. You do not know the legitimate quality of what you are injecting and can develop many problems following this. 
An example I can give is the use of non medical grade silicone in PRP breast implants which resulted in a number of patients having to have these implants removed. The implants were made with non-medical grade silicone and therefore were cheaper. This subsequently meant more expenditure and down time for the patients.
It is your prerogative as to risking this - but even in the best hands there can be complications. As a surgeon, I would never take on a procedure, if I was not 100 % confident that I could deal with all the complications that can arise. If you are considering injecting filler into your lips or cheeks - how deep will you go? what if you inadvertantly inject into the blood vessel of the lip? around the cheek - what if you are in the wrong plane and inject around the vessels of the eye. There have been cases of incorrect administration of fillers and fat injections around the eye that have lead to blindness.  
It is a very important question that I am glad you have asked and I must say that I would stay well away from trying to carry out these procedures yourself without any medical training. Most of us have been trained for a very long time and continue to train and re-train ourselves. I have 17 years experience in the field of medicine and plastic and reconstructive surgery and even now I an still cautious when carrying out procedures/ treatments etc as you must remember - we are treating the human body and  it can react to our interventions in any way. Things normally go right, but when they go wrong - they can go very wrong. 
Hopefully this will help you to decide not to do this.
Kind regards 
Rohit Seth