Glad to hear that Mr Miyagi is feeling better.
Lactulose does require a prescription from your vet. Many people have used slippery elm bark that you can get at your health food store. There is some information about constipated cats on this website ....
http://www.littlebigcat.com/index.ph...onstipatedcats
And, the link to slippery elm bark ....
http://www.littlebigcat.com/?action=...em=slipperyelm