Right this is a very difficult area that places both the vet and the person with the dart gun under a lot of scrutiny and there is the potential for people to die if these drugs are used irresponsibly. A Veterinary Surgeon can only prescribe drugs for animals under their care, so your local pet vet can't just supply you with Immobilon as you need it. As an absolute minimum the vet prescribing will have to have visited the area and carried out a clinical assessment of the animals (different to an actual physical examination). The vet's responsibility carries forwards when the drugs are used and if someone died they have a degree of culpability.
The very simple answer is nobody wants the hassle or the risk, so very few vets will prescribe the drugs for you.
The rules we work to are here and set by our professional body - the Royal College of Veterinary Surgeons
Guide to Professional Conduct for Veterinary Surgeons - RCVS
Personally if drugs like these are being used I think it should be in vets hands, or at least have a vet in attendance. These are powerful, dangerous sedatives and anaesthetics we are talking about here. 1 drop splashed in your eye can kill you. I'm not being alarmist.