I know where you can get iso-muscone - Perfumerís Apprentice have it - but so far as I know l-muscone is only made by Takasago and although I got a sample from them
a) it took months as it came by surface transport from Japan and
b) to buy any you have to meet a huge minimum order.
Currently I only use it in very special commissions because all I have is a little sample. The difference between the two is very subtle though: iso-muscone is a very good alternative.
If anyone knows of another source Iíd be interested to know too.