Forget trying to replace the buttons, or the cases. The buttons on the originals are "welded" to the case on the inside. The replacements off ebay will just float around inside the case. Useless. Plus you'll be hard pressed to get the original keys apart without destroying the case (it's ultrasonically welded together)
You can buy replacement cases off ebay (non-genuine of course), but they don't work very well. Besides, the internal battery will almost certainly be shot anyway (or close to)
I got my first replacement key (including blade) from Mr Minit, I think it was about $150, and I'm almost certain that they used genuine Holden parts.
I got two further genuine uncoded electronic 'heads' off Gumtree, for about $80 (the guy worked for a Holden spare parts...I asked no questions...), then had them coded for ~$70 the pair at the local key cutting place in North Rocks. I re-used the original blades. Unless the blades of your existing keys are worn (doubtful!) you really only need the heads, I reckon. Dunno what Holden charge, but Holmart has genuine ones for $68 each:
http://holmart.com.au/online/prod407.htmWord of warning: Don't get to the point where you have no working keys, as I believe the locksmiths use the existing key to clone the new one. I would think that if they can't do this, you
would need to go to Holden and pay whatever they say...
On a related note: if your key is getting sticky/reluctant to turn in the key barrel, do yourself a favour and get a new barrel before it jams up completely. Only about $30 and an hour or so to replace (I did mine myself but it is a bit fiddly...)
Cheers - Andrew