I use decent Scottish salmon or trout. If you want to try worms go to a fishing shop and ask for a tub of lob worms.

BTW it's not strictly true that feeding live fish is illegal, what is illegal is causing unnecessary suffering to an animal. In the case of a carnivorous animal that will not take pre killed prey it's a grey area. Not feeding the snake would be causing unnecessary suffering to the snake, it naturally eats fish, so there is some defence. I don't believe a case has gone to court where the live food was necessary to prevent any animal from starving. More to the point though, you can't walk into a shop and buy feeder guppies...