I was able to order a single beef heart from a local butcher. So if you have any butchers or specialty meat markets, that might be an angle for you.

Keep in mind that Discus are "grazers" and it may take them a while to eat a particular food vs. gobbling it up right away. I also tried the beef heart flakes and my discus would not eat it until the 3rd or 4th time I fed it to them. If you feed flake or pellet first thing in the morning when they are hungriest, that might also encourage them to be a bit more "adventurous" with their palate.