In no particular order. Feel free to add your own.
don't be afraid of your VW dealer - you'd be surprised how cheap some parts are - and nothing beats genuine parts.
2. Classic VW - http://www.classic-vw.co.uk
This is a one man operation run by a VW enthusiast. A huge selection of items - and many obsolete elsewhere. If it's not in stock, email or find him on Facebook.
2. VW Heritage - https://www.vwheritage.com
Excellent for a lot of quality aftermarket parts - mainly Meyle but also peppered with a few genuine items here and there. Great, reliable service and staffed by folk who know/like/are interested in VWs. They also have an ebay shop if you want to earn Nectar points...
3. VW Classic Parts - https://www.volkswagen-classic-parts.de ... rocco.html
Rip off postage for small orders but again, nothing beats genuine. Some stuff crazy cheap - complete passenger side mk1 Scirocco mirror for 19 Euros. Always check the part number in VW Heritage's "Classic Parts" search as it may end up cheaper.
4. Halfords - http://www.halfords.com
For this to be worthwhile you need a Trade Card - or have a friend with one. At least 20% discount off Halfords branded car related items - everything from Spray Cans to Bulbs, Wipers and Batteries - some bulbs and other bits can be over 50% off. Obviously tools are discounted as well - and usually beat even the biggest sale discount.
5. GSF - https://www.gsfcarparts.com
not as good as they used to be but there are 100s of branches and every weekend there is a 50+ % discount code. Sign up for their emails. They also have an ebay shop.
6. ECP - https://www.eurocarparts.com
see above. Not great on the discount codes but often great savings to be had. Better at stocking premium brands like Bosch than GSF.
They also have a subsidiary https://www.carparts4less.co.uk
where the same parts can sometimes be found cheaper. They both have ebay shops too - and that can sometimes have the best price/.
7. Amazon - https://www.amazon.co.uk
Great for prices on things like Brembo and Febi and next day delivery for a lot of stuff if you have Prime. Tip - make sure you are buying from Amazon themselves and not a reseller.
8. B+B Components on ebay - http://stores.ebay.co.uk/tierod69/
Great for a variety of parts and some pretty obscure ones too - at good prices and always quick despatch - I've used them on scores of occasions. Just click and search Scirocco etc..
There are a few of these that will pop up at the top of any Google search. I've bought from the above ones - a couple of which I believe may be the same company. A huge choice of brands for any given part. Shipping is from the EU so will take a few days.