Depends on what you want/need.
For general service/repair parts, Autovaux in the UK are the way to go (still cheaper than Holden), www.autovaux.co.uk
For some more custom parts, often helpful with higher end performance upgrades and in the infamous short shifters (which are highly recommended by the way), check out M-Tech at www.opel-tuning.nl
Need new brakes? GSL Rallysport/Brakes Direct are not only forum sponsors, but they're great to deal with. www.brakesdirect.com.au
LMF Vauxhall (www.lmfvauxhall.co.uk), aren't bad, but their shipping can be stupidly expensive.
Custom instrument dial sets like mine can be found at www.xtuners.com Only thing is with this guy, he can take his time getting them to you, but the quality is top notch.
For LEDs, I used Crazyleds (www.crazyleds.co.uk). Personally, I have not had any issues with mine, but there are some that have.
That's all I can think of off the top of my head at the moment.