It is possible for heroin to be taken rectally, but it's not something you'd want to do too often as it is too unpredictable. Usually, if your thinking about doing this, you've more than likely gone past the experimentation stage and you'd be looking for value for money i.e I
Morphine suppossitories do exist but are not marketed in Aust. but are available in America and to the best of my knowledge, in the UK. If you do find some, you might find they cause a pressure in your head which can be kind of unpleasant.
Just like to add without wanting to appear preachy, that the ultimate misery of heroin far outweighs the initial bliss. Think carefully.