Yes its hard enough getting hold of the bits and peices in the first place over here....
You can usually pick up joysticks and buttons off ebay quite cheap, other bits are hard to come buy. Also you can often find an old control panel with buttons/joysticks intact....
To be honest, the type of stuff people will occaisonally have spare (buttons/the odd joystick) are actually pretty cheap. I threw together a list of suppliers a year ago here:
http://www.silverfoxy.plus.com/UKinfo.htmlIts slightly out of date (mainly that HAPP UK won't help you now).Some are international but will ship to the UK. I found that several items were cheaper to get shipped from the US.
I got a lot of parts off Happ, but now they've 'merged' with Suzo I don't think they do small orders anymore.
Ultimarc (maker of IPAC) is UK based, and sells buttons/joysticks and stuff, and theres a couple of others. Swallow Amusments sell new and 2nd parts.