I buy my onesies from Ebay, the brand I buy is Carters (an american one). They go up to size 2 and come in packs of 7 at a very reasonable price with postage included. If you type in onesie on Ebay and look at the OS sellers, they have fantastic onesies. The UK also have good ones.
For some reason, Australia doesn't seem to have a great range of reasonable priced ones, the Bonds one usually go for $7each and also Marquise (available through Myer) make them up to size 2 as well (excellent quality) again expensive per item at usually $10 perhaps a little less? The Carters ones I got from the US came in a pack of 7 and only cost $20 Aus with postage included.
Best and Less and BigW are usually ok for onesies as well.