I got the referral for an extra u/s from my GP for m/c. It was $93 (but i think they are normally $108??) and because I had clinical notes on the bottom (bleeding and pregnant) I got my medicare rebate of $50odd.
I should imagine if you are prepared to pay for one, and not get all of it back, your GP should be able to refer you. It may be more if its later in pregnancy.
Its a crappy way to handle it - but this is the way the public system is I guess.

Fi