Just type into the eBay search bar : 5x100 wheel spacers and find hundreds of different ones. About 8mm is the maximum you want to go on each hub without using hubcentric spacers and longer wheel bolts. 15-20mm is commonly used with standard GTI longbeach alloy wheels but as mentioned it all depends on how low your car is and what tyre profile you are running.
Personally i'd put 10-12mm on the front and 15mm on the rear, all hubcentric with the longer correct wheel bolts.