Hi Emmaline,
Focus on businesses that are selling opal to the trade or local population. International sellers or tourist markets are most likely to have elevated prices
Gemshows are a good place to start. There's usually one a month in our larger cities. Go have a look and compare prices. Gain an understanding of what types and colours are most valuable.
Unless you know the seller I'd generally avoid buying opal online, its unlikely to look like its photo and may be prone to cracking, even after the Paypal claims period is over.
I'll PM you a couple of business card details from a few opal dealers I met at a show. That might point you in the right direction.
Cheers,
GR