I used to buy whole beef hearts from the local grocery chain (Cub Foods). Just had to call the meat department so they can order it. However, you'll need to hack away all the membranes, blood vessels, fat, etc., which removes about 1/3rd to 1/2 of the total heart. I ground the rest in a food processor. So it's not a matter of asking your butcher to just grind up a beef heart. I found it a lot easier to pay the price for ground beefheart.
The only reason beef heart was used was because it had the lowest fat content. Today, there is extra lean ground beef that you can use. The Excel brand comes in a roll and is available as 93% lean.
I only use bloodworms as the last feeding of the day or if the fish go on a hunger strike. It doesn't provide much nutrition at all.
Willie