Various Tuners found in GT3, you could buy parts from some, and other just have a racing car represented!

AMG (Mercedes-Benz Modification Company)
C-West (Modefies cars for many companies)
D.R.S. (Daihatsu Modification Company)
Ford Racing (SVT/Ford Modification Company)
Masterfit (Vauxhall/Opel Modification Company)
Mazdaspeed (Mazda Modification Company)
Mine's (Modefies cars for various companies)
Mopar Parts (DaimlerChrysler/Dodge Modification Company)
Mugen (Mugen-Honda Modification Company)
Nismo (Nissan Motorsports)
Ralliart (Mitsubishi Modification Company)
RUF (Porsche Tuning Company)
SiFo (Renault Tuning Company)
Spoon Sports (Honda Modification Company)
STi (Subaru Tecnica International)
Suzuki Sport (Suzuki Modifining Company)
Tom's Racing (Toyota Modification Company)
TRD (Toyota Racing Development)