The first step is actually not posting here.
You might actually want to try searching in various places for what you want.
You can try Google: http://www.google.co.uk" onclick="window.open(this.href);return false; to start with
If you want second hand parts:
http://www.ebay.co.uk for spares or breaking cars.
http://www.fordpartsuk.co.uk or Google for breakers
Your local scrappy/paper/ad mag.
If you want new parts, it depends on what you want.
There's http://www.pumabuild.co.uk and http://www.pumaspeed.co.uk are good, but can be expensive.
http://www.camskill.co.uk or http://www.blackcircles.com are good for new tyres for good prices.
There are numerous other sites for wheels, exhausts modifications etc. If you want recommendations for sites/sellers that people have used before, then all you need to do is ask in the appropriate section of the forum.
Then have a look through our For Sale section, there's always a few people with bits and bobs that they're selling.
Then, if you feel you've reasonably searched everywhere you can, and still can't find what you're after, post here, and see if someone has that elusive item in their shed/loft/spare room.
Remember, this forum is not for those people who can't be bothered to do the legwork themselves and see it as an easy option. Double posts (i.e the same thread here and in other sections of the forum) will be removed.
Remember to include how much you're willing to pay, whether you're willing to collect or pay postage etc, it'll help the people who might be tempted to offer whatever they have.