Depending on where you are in Australia, you should be able to find an electronics store like jaycar if you're in the big smoke. Normally speaking, 5M ledstrip itself can powered from 1 spot at the beginning, or at least it is advertised that way, and for 300 leds you do need quite a big PSU, although i found that something a bit smaller than the 18A calculated will also do unless you intend to run the ledstrip at white full brightness.
For injecting power determination, power the strip at 1 end, and make the leds 'white' If you see a discolouration towards brown at the end of the strip, then you should inject power halfway somewhere as well,