You might try searching for something similar in Australia to 'Radio Locator'. That site is mostly US based and while I can search Australian channels, the site will not show me vacant Australian channels as it does for US channels.
I would park out the front of your house and scan the frequency range looking for an "empty" frequency. (Doing it a few nights around show time as some broadcasters seem to come and go around my place)
And what sort of issue are you having? Could it be your transmitter is the issue or even the quality of the original signal, I had to add a Behringer mixer to my setup to improve my audio quality.
Has anyone experienced poor audio quality from their Pi this year? Last year with the same setup and same mp3 tracks, i had good audio quality being transmitted. I could pick it up on all my radios and it sounded great. This year though, I have tried adjusting the gain of the transmitter and...
Also check cables as I have had a faulty 3.5mm cable before. Upgraded to a SoundBlaster 3 soundcard last year also to try and improve. I chose an even number the first year and have just stuck with it, Each year I think I should change but it is always down the bottom of priority list and gets forgotten.
I installed a Sound Blaster Paly! 3 USB to my laptop on the weekend to see if it would improve sound to my FM Tx.
Here's some things I found during testing (with an FM radio a meter or so away from TX):
With no audio playing, to check for hum and hiss:
Volume out of laptop no higher than 70%
Line In "volume" on FM Tx to 100%
Mic In "volume" on FM Tx to 80% - this was the real odd one to me as it shouldn't affect anything as nothing is being fed in! But there was definitely more hum as I turned the Mic In up and down
With audio playing:
settings left as above, sound seemed ok.
However when I moved the radio further away I noticed some clipping and distortion. So I dropped the Laptop volume down further and got nice clean sound