to find out which engine you have and all options the car was originally ordered, use the last 7 numbers of your car VIN here and you get all the details
BMWVin.com
Enter last 7 characters of your BMW VIN, our VIN decoding service will analyze your vehicle's serial number and generate a report that describes the information the VIN contains. This VIN decoder can extract such information as : year, make, model, trim, engine, transmission,manufacturer, country of origin and options.

Also when you want to know if a part was also used in other BMW odels, use RealOEM.com Online BMW Parts Catalog and search by the VIn for the correct part number you need, then return to page one of realoem and use that p/n and paste it into search by part number and all BMW which use that number will pop up. That is fr example good when you want to check for used parts.
As I live in Japan and prices are also as high as in Australia for the parts, I buy a lot from USA, even with shipping cost it is still cheaper than here.
But as you have RHD drive, be careful with parts numbers, also a lot of other parts will not fit your AUS spec RHD BMW, as they are USA spec cars. Some links
USA
Auto Parts at AutohausAZ - OEM Auto Parts - Discount Replacement Parts, Resources and Car Care Tips
BMA Auto Parts | Bumper to Bumper since 1978 | Oem auto parts, vw parts, sachs boge, mercedes benz parts, honda auto parts, saab parts
Pelican Parts - Porsche Parts & BMW Parts - Automotive Parts and Accessories - Porsche & BMW
all of them ship overseas, use or select in their online order forum USPS priority parcel or something like that, USPS takes a bit lomger than FEDEX or DHL but is much cheaper.

U.K.
CAR PARTS from Mister Auto - Your Parts at discount prices
Cheap Bmw Car Parts From 247 Spares Save Up To 80% NOW
BMW Car Parts, Spares, Accessories, Motormec, UK
BMW Parts From BMW Parts Online

there are of course more, just some listed here.