Forget trying to replace the buttons, or the cases. The buttons on the original are “welded” to the case on the inside. The replacements off auction web sites will just float around inside the case. Useless. As well as you’ll be challenged to get the original secrets apart without destroying the situation (it’s ultrasonically welded together)
You can buy alternative cases off ebay (non-genuine of course), nonetheless they may work perfectly. Besides, the internal battery will likely be shot anyway (or near 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.
We got two further genuine uncoded electronic ‘heads’ off Gumtree, for about $80 (the guy worked for a Holden spare elements… Specialists no questions… ), then had them coded for ~$70 the pair at the local key cutting devote North Rocks. I re-used the original blades. Unless the blades of your existing secrets are worn (doubtful! ) you really only need the heads, I reckon. Dunno what Holden cost, but Holmart has genuine ones for $68 each: http://locksmithnorthbrisbane.com/
Word of alert: Do not get to the point where you have no working keys, as I actually believe the locksmiths use the existing step to clone the new one. I would feel that if they can’t do this, you would need to go to Holden and pay whatever there is a saying…
On a related note: if your key is getting sticky/reluctant to turn in the key barrel, do yourself a favour and obtain a new barrel before it jams up completely. Just about $30 and an hour or so to replace (I did mine myself but it is somewhat fiddly… )