Although I am not sure of a store, online or otherwise, where you can buy an anatomically correct heart mold, one approach you might consider is to make your own mold for baking.
You could buy polymer baking clay to make the mold. As long as you don't heat the mold up, you don't need to worry fumes. However, a standard precaution for Sculpey brand is to use dishes made from their clay as decorative only. Therefore you will need to create a barrier using another inert material; wax, saran wrap? Or you could seek out a sculptor and get an earthenware heart mold.
Another idea is silicone. I am not sure where you'll be able to procure silicone in teeny tiny bulk in Australia (not that I know where to buy it in the US either), but I have seen online people casting silicone using modeling clay.
Basically, if you were to follow up on this, you would likely be using a Dow Corning "Silastic"; here is a link to XIAMETER® RBB-6630-30 BASE, which is inert and compliant with high thermal, repeat use per 21 CFR 177.2600 (U.S. Food & Drug Administration). There are additional products available in Australia, so if you cannot find a distributor for that one you might try another.
Yes, I know the latter option sounds over the top, but I thought I might throw it out there.