The most common centre cap on an Audi TT is one from the 18" 9 spoke, or RS4, wheels. This centre caps clip into the centre of the wheel and cover the wheel bolts.
The part number for these caps is 8DO601165K and main Audi dealer price is £25.00 + VAT each, quite expensive.
You can get new or nearly new replacement Audi TT centre caps on ebay for a fraction of the price of new ones.
If you have replica RS4 wheels, the genuine centre caps will not fit in them. You will need to measure the cap and compare it to caps that are on sale, but it very hard to find ones that fit.
Check the listings below to find Audi TT centre caps that are cheaper than a main dealer:
Part Number: 8DO601165K
Other Part Number: 8DO 601 165K
Colour: Brushed Aluminium Finish, Gunmetal Silver centre