Not sure when it is available from but Alderney Electricity are supplying it and they have a number for AVGAS so I suggest you give them a call.
Alderney Electricity Ltd
Avgas
(01481) 824666
Nice Golf course up there and plenty of friendly hospitality.
If you stay for food Nellie Gray's was good for curry last time I was there or you could try Georgian House for excellent Steaks & traditional food.
Have fun.
