Audi tuning boxes from a variety of manufacturers on ebay. It is important that the tuning box you are looking at is adjustable, you want to be sure you are getting the most possible power.
A nice automotive type connector (as opposed to a computer based connector) is also a bonus. We avoid tuning boxes which have bluetooth functionality, the potential for problems is not outweighed by the features. Generally when you have found the setting you like there is no need to adjust the tuning box later unless the engine is under extreme conditions, eg track day, towing. A long warranty is also nice to have on the product. We do not put much stock in engine and gearbox warranties, it is almost impossible to tell if damage is down to the tuning box, the condition of the engine, the driver etc. If you have find a tuning box with TUV certification, it is a bonus. TUV certification would suggest the power claims of the tuning box manufacturer has been tested independently. TUV approved tuning boxes quote smaller power gains than non UV approved boxes. Ill let you draw your own conclusion about this.