Beef heart is easy to get ahold of at any local supermarket or butchers market, you just have to ask the butcher specifically for it, they generally don't have it on the shelves.
gwynbrook farms discus has an excellant recipe that I made for my juvies and they love it. but it's some kind of messy , you dirty for the tank. but it good to mix it with other foods. take a look it can't hurt.