I mainly buy on Ebay especially products in 5 litres as it nearly always works out cheaper per litre. I have also started popping into The Detailing Shack https://thedetailingshack.co.uk , only 10 minutes from me and always does me a good price if I buy 2 or 3 things at the same time
-Slims
-Cleanyourcar
-Find the product on eurocarparts, copy the product code, search for the code on carparts4less (find a discount code) and sit back in the satisfaction that I have bought a product for a decent price (usually there is a 18% code about every other week).
-Ebay