-know someone with a connection (or work in the industry)
-eBay: but while searching for deals, also search Google for shopping results, you may be surprised as some online shops are cheaper than eBay sometimes.
-the "For Sale" section on this site.
-your local for sale adverts (Craigslist is popular in North America, not sure if it's as strong in the UK or if there is an equivalent)
Buying used can be fine, by the way, as long as you trust the seller and review the condition thoroughly.
Worldwide distribution is interesting.
I have listed the following price points from official dealers.
Enve in Thailand is about 5% cheaper than Enve in the US.
Shimano Thailand is about 10-20% cheaper than Shimano USA.
Zipp is about 20% cheaper than in the USA.
SRAM Thailand imports directly from SRAM Taiwan and is 30% cheaper than SRAM USA.
Lezyne is also 30% cheaper than Lezyne in USA.
Assos is almost 30% cheaper in Thailand, Italy, and the UK.
Fully assembled bikes from Cannondale, Cervelo, Colnago, Specialized, and Trek are about 20-30% more expensive here but Giant is cheaper.
Thus, custom built bikes here cost about the same as fully built and imported bikes. But, there are no major sales here apart from the occasional close outs.
From grey market, direct importers, minus about 10% but no warranty. I suppose this would be like buying from eBay.
So, whenever I see a 20-30% discount in the US, with free shipping and no sales tax, it is a good deal.
Also, the end of season clothing 50% off sales in the US can not be beat.