Any of the hobby suppliers should be able to order it for you from Danisco. My Danisco contact in Germany has told me the DSM product is actually theirs, DSM buys it off them. Makes sense as DSM can't match their price for it, but it is really expensive. Good thing about it is the recommended dose in solution is about 5ppm, so that $75 will last a while. I think we get it in 100 gram bottles, it's a powder.
And yes, natamycin will kill blue. It generally is good for about 4 weeks, then starts to degrade.
Oh, check it's other name: pimaracin